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Not padding the map with about 3x as much generic copypasta wilderness as it had actual content would be a good start.
There are even better examples. Let's just take a look at the Gamebanshee walkthrough for BG1 and pick a wilderness area.
http://www.gamebanshee.com/baldursgate/walkthrough/ar4300.php
The walkthroughs over on Gamebanshee always show all the points of interest on any particular area map, neatly labeled. You can count the amount of points of interests on half a hand here. There's three interesting things to discover on a huge map with lots of empy space in between. Exploration on this map consists of walking through an empty landscape, and if you're lucky you encounter small groups of 3 hobgoblins to fight, and then these 3 points of interest. Wow. Such excitement.
In fact, let's check what these points of interest even are:
1. Get ambushed by hobgoblins and find dead travelers with an amulet you can return to their father for a reward
2. A fourth wall breaking character advertising the company's next franchise (since NWN was released only after BG2, it's possible this character was only implemented in Beamdog's Enhanced Edition - if that's the case, the original version of this map would only have two points of interest)
3. A hermit who tells you something inconsequential in a small, irrelevant tongue-in-cheek conversation
So... of these 3 points of interest, only one is actually interesting: you get ambushed and find two dead people who have a quest-relevant item on their corpses. The other two are pointless fluff, and they're not even the good kind of pointless fluff. They just ramble at you with conversations that have no relation to the game's story and that's it. And the ambush is right at the point where you enter the map, since when you first enter it you arrive from the north, so you can't miss it. You'll walk right into it first thing you do.
So the interesting content on this map doesn't even need to be discovered. It comes at you right away, leaving you then with a map that is completely empty and doesn't have anything left to discover.
Yeah, peak level design right there.
To be fair to the game, let's take a look at some other areas. This one might just be an exception. I've picked a wilderness map from the Gamebanshee walkthrough randomly, and found this:
http://www.gamebanshee.com/baldursgate/walkthrough/ar3000.php
Again, it's a huge wilderness map with only 4 points of interest. Let's see what they are:
1. Some dude wants you to retrieve a scroll that was stolen from him. It's at point 1a.
2. Red wizards of Thay talk to you, then inevitably attack you, leading to a challenging encounter.
3. A bunch of spiders attack you, leading to a challenging encounter.
So... one pretty generic fetch quest (a monster stole a thing I own! Bring it back!) which is repeated throughout the game several times. Seriously, most of BG1's sidequests follow the pattern of "A monster stole a thing I own! Get it back!" so this is just one more quest of the type you already solved a dozen times before. Yawn.
Then there's also one genuinely interesting encounter with a bunch of evil wizards.
And then there's a relatively generic encounter with a bunch of spiders.
It's... more than the other map had, I guess. But there's still plenty of empty space, the encounters are all concentrated to the northeast section of the map rather than scattered throughout, meaning most of the area you explore doesn't contain anything interesting. And only one of these encounters could be called interesting at all.
Ok one more just for fun:
http://www.gamebanshee.com/baldursgate/walkthrough/ar4700.php
Xvart village! Ooh, finally something interesting... or is there? Hey, look at all that empty space at the bottom. The map could have been cut in half vertically, and nothing of value would have been lost. The areas beyond the central mountains are completely empty and devoid of content.
As for the content itself...
1. A village of Xvarts that attacks you on sight. Challenging with a level 1 party, but already becomes rather trivial at level 3 and above. No interesting loot to find except for generic weapons, gold, and low-value gems.
2. One Xvart in the village will call a bear that will attack you. Challenging at low level, adding some variation to the Xvart village.
3. A tiny little cave that contains a treasure chest and one bear just like the one you just fought in the village, except here the bear is alone so the encounter is comparatively trivial.
4. Some random guy who wants to scam you by selling you fake magic items.
It's... ok I guess? Better than the previous maps but still very generic and meh overall. Also, look at all that empty landscape south of the mountains! There's nothing there! Nothing whatsoever!
Also, this is a good time to point at the inconsistency of BG1's world design. Most of these locations have nonsensically wacky encounters in them: some nobleman who tells you about Neverwinter Nights, a hobo who rambles at you with random stuff, a guy who tries to scam you with fake magic items... all of these wouldn't be bad encounters in and by themselves. They'd be great little encounters to add life to a city like Baldur's Gate. But they're not in a city. They're in the wilderness. Random places of the wilderness. Just standing there, among the trees, waiting for an adventurer to encounter them. Ok. Yes. Very immersive, very believable.
And that's why BG1's exploration is bad. You have large maps that are largely empty and filled with maybe 1 to 3 points of interest. Those points of interest are either generic as fuck (a group of hobgoblins that attack you! a guy who's had his scroll stolen! a cave with one bear in it!) or so random they don't make any sense being there (a bunch of pretty powerful wizards standing in a random place in the middle of the wilderness with nothing nearby that would be of interest to them! a guy trying to scam you with fake magic items somewhere at the ass end of the world where usually no travellers pass by!).
It's not very coherent. It's just a mess of at best mildly interesting things with little consequence to the big picture. Even if the exploration itself was fun at a systemic level, there's not much to discover to make it worthwhile.
And before anyone says "But Jarl, you like Morrowind, don't you? That also has plenty of empty areas to traverse and large stretches of land that contain nothing more than a random cliffracer encounter or a boring NPC that gives you an escort quest!", yes, Morrowind also has plenty of empty land and the generic quests - especially the escort quests - are terrible and one of the things Morrowind doesn't do well at all. But the exploration itself is more fun because you're taking a more active part in it. You explore a fully 3D world in first person perspective, you can jump and swim and dive and even fly if you have the right spells for it. You have actual spatial navigation as a core gameplay element. I love playing Morrowind with that mod that radically increases the view distance, because you can look at the horizon and see a faraway island and you can just decide to swim there and explore it. Or you look at the peak of a mountain, and you can climb it. Navigation is a gameplay element in itself.
In BG, exploration is a matter of "comb over the entire map and uncover all the fog of war, until everything has been discovered".
That's not very engaging, is it? In fact, you could just use a cheat to reveal the fog of war and wouldn't lose any gameplay by it. You'd just avoid tedium because that cheat will show you the large empty stretches of nothingness, so you can just ignore them instead of exploring them in the hope of discovering something that isn't there.
Let's contrast this to BG2, which removed the empty and boring wilderness areas and created a more focused world instead, where each location was connected with one or more quests and had some actual design effort put into it.
http://www.gamebanshee.com/baldursgateii/walkthrough/umarhills.php
This is Umar Hills, one of the areas outside Athkathla where you can follow one major quest and have a couple of side quests, too. Look at the arrangement of the points of interest: most of them are in the southeast, where the vilalge is - understandably. But there's also one point of interest to the far north of the village, one point of interest in the southwest corner, and one in the northwest corner. Every corner of the map has something interesting in it. While there is some empty-ish wilderness space in the center, the map doesn't have any excess landmass that could have been cut: if you look at the BG1 wilderness areas I posted above, they all have large stretches of nothingness at the edges of the map. Not so here. The content in Umar Hills is spread in a way that guarantees you will find something at every corner of the map, unlike, say, the Xvart village in BG1 where the entire southern half of the map is completely empty and devoid of content.
And what about the content itself?
1. You arrive here and the mayor addresses the citizens about recent cases of murders and disappearances. Nice, sounds like a quest hook that actually has something to do with the local village, rather than BG1's generic "a monster stole my item please bring it back" shtick.
2. You can find a merchant near the inn who turns out to be an accomplice to the murdering tanner from another quest, set in the Bridge District of Athkathla. This immediately establishes that the world is an actual world, where characters in one place may have connections to characters in other places. Much, much better than BG1's many instances of random characters appearing in random places and having nothing at all to do with anything else in the game.
3. The minister will ask you to investigate the ogres in the north and ask them if they have anything to do with the disappearances of villagers. Nice, the hook that was established at arrival is taken up and you get a quest that actually has something to do with local events!
4. Some random sidequests, but they're all more involved than what you get in BG1. Children who want to be adventurers and ask for swords, and if you give them to them the mayor will tell you you're no longer welcome in the village for endangering the children. A quest where a wizard wants you to dissuade his daughter from seeing her lover, but if you can make him change his mind by saving him from a botched golem construction. A guy who tells you one of the farmer's chickens has swallowed a valuable gem. They're small quests, similar to a lot of the quests you get in BG1, but they add some flavor to the village and they're at least marginally more interesting than the "monster stole item plz bring back" type of quest.
5. The house of the local ranger who was killed by whatever it is that kills people in this town. You find a journal here that leads you to another location. Cool. That means this location isn't just self-contained but just part of a quest that spans several locations. Much more interconnected than any of the generic wilderness areas of BG1.
6. A location related to one of the quests at point 4, just a simple "enter cave and fetch item" affair, so this particular one is about on par with BG1's points of interest.
7. A ranger who can become your companion and tells you about the plans of the cowled wizards to enter the planar sphere in Athkathla. Oh hey wow, is that another reference to a different quest in a different area of the game? Looks like in BG2, things are actually all somehow connected with each other, rather than being random one-offs with no consequence towards anything!
8. The ogres you're supposed to talk to in the quest you were given at point 3. You can either kill them all, or talk to them and they'll tell you they're not responsible for the murders, instead they actually want to establish trade with the village. Cool.
So, just in this one location we have about as many points of interest as in two of BG1's generic wilderness maps combined, and every single point of interest is more interesting than the BG1 ones because they're somehow connected to a quest and have a larger context to their existence rather than just being there for the sake of being there.
Let's look at another BG2 area:
http://www.gamebanshee.com/baldursgateii/walkthrough/windspearhills.php
Now, at first glance this doesn't look that much more interesting than the BG1 wilderness areas, does it? Only 4 points of interest, and plenty of empty space in the center.
But let's look at the actual points of interest:
1. You arrive here and are immediately attacked by monsters, but it turns out they were actually knights with an illusion spell put on them to make them appear like monsters. A guy arrives and takes you to his home at point 2 to talk to you about this.
2. The guy explains to you that a guy named Firkraag stole these lands from him and terrorizes them with his illusion magic. Also, his daughter gets kidnapped by bandits while you visit him! Boom, instant plot hook for a quest. The guy wants to you get his child back and deal with Firkraag if you can.
3. You can find the feary queen here to whom you can give some acorns you received from dryads in the prologue. This area lets you conclude a quest you've started in the prolouge and haven't had a chance to finish until now. Cool!
4. This leads into the dungeon where you'll deal with the quests you received at point 2.
So, overall fewer points of interest compared to Umar Hills, but still vastly more interesting than BG1's wilderness maps because the content you find here isn't just a bunch of random NPCs spouting random nonsense, or random encounters with generic enemies that have no particular reason to be there. Instead, everything is connected to some quest or another, there's instant drama and story related to everything you encounter here, and it actually manages to pull you in narratively.
BG2's overall area and quest design is just so infinitely superior to BG1's, it's not even funny.
And I haven't even posted one of Athkathla's districts yet, which are the densest and most interesting areas in the game.