BG is much smaller than Morrowind. This means that while in Morrowind you have to decide what to explore, in BG you can, and will 'explore' everything.
There's no decision of what to explore or not explore in Morrowind; the player has as much time as they wish to systematically search every square area of land. Time or resource constraints are non-existent in the gameworld. Only the player's time/patience serves as a limiting factor.
This leads into what I feel is the most salient critique of BG1's exploration, which also applies strongly to Morrowind (as well as many other open world games). Both force the player to gamble away their time on the Content-Roulette, with some wins and a lot of harsh losses.
In BG1, there are some areas that are, simply put, total shit. They contain nothing unique nor interesting and serve only as filler that wastes the player's time. Take for instance the locale in the extreme southeast. Gibberlings abound and not much else besides a lousy Fed-Ex quest. But the unknowing player will still go to clear it and likely leave underwhelmed, at least in the back of their mind. There's definitely more areas like that. Not every place has a real hook or hooks.
Exploring places, especially dungeons, in Morrowind is a not too dissimilar affair. For every rare occasion that the player is rewarded with an object of interest, be it a piece of equipment or some lore pellet, they are inundated with meaningless, bland dungeons or empty terrain. Rooting around indoors will yield a couple thousand finds of lesser soul gems (or similar trash items) to every one time a Daedric Killmaster 3000 is wedged in between a couple of boxes. It's a bad distribution of goodies; 99% of the time stuff is crap, while 1% of the time it's outstanding. The observant player realizes that meticulous exploration is important, but will only be rewarding a scant minority of the time. Terrible incentive structuring, terrible design for a game. Compare to something like Risen or Dark Souls, which have a lot more meaningful finds delivered more often. This makes their respective Content-Roulettes have a much better expected value, encouraging scouring the gameworld.
And neither world is very good at flagging areas as crap/not-crap.
BG1 names some of it's areas, but everything else is just guesswork. Can any information about an area be gleaned from 40x40 pixel sketches representing areas? Does a cottage under a tree adequately convey, "Here be Basilisks, mad mages, and a party of rival adventurers"? Even named areas aren't a flag for good content. The Gnoll Stronghold, while having plenty of in-game reasons to visit, is a tad underwhelming in gameplay. A bunch of high AC/THAC0 meleed00ds does not make for a compelling gameplay session, even if picking up Gauntlets of Dexterity, a charisma tome, and/or Dynaheir justifies the trip. Now if it was filled with casters, archers, and maybe some domesticated critters acting as guards...well,now we're talking.
Morrowind is pretty bad in this regard as well. Any given dungeon could be naught but a bunch of trash mobs and crap loot; in fact, many are nothing but. Even "unique" dungeons lack interesting gameplay content, mostly a fault of Elder Scrolls shoddy core mechanics. Loot and lore are basically pellets that keep the player on a (very boring) hamster wheel.
Basically, it's the inconsistency of content quality (usually with lots of mediocre/bad) that make "exploration" bad, not being restricted to a 2d environment.