Well, first of all, keep in mind that Marathon 2 is a true sequel. As in: the game with basically the same combat mechanics and very similar gamedesign. On a plus side, Marathon 2: Durandal is probably as far as you could polish this particular turd.
First of all, the controls in M2 were much more fluid and responsive (I am talking about Win95 version of M2 compared to Basilisk2-emulated Mac version of M1), there is less inertia, and moreover, for the majority of the game you a have kind of a normal gravity, so the fighting upon the rugged terrain is now at least somewhat tolerable. The motion detector also works much better now and is actually quite useful.
The weapons stayed more or less the same (with the addition of one new gun and a complete remake of another gun you haven't seen anyway). Some of the most annoying/useless enemies from the first game (wasps and hulks/drinniols, maybe also some others) are also gone - and a fair bunch of others got introduced. All the enemies and guns were remodeled - and, moreover, guns now have sounds which are actually quite nice and well done.
Remember how the first game was this collection of grey and brown walls, etc? Well, the second one swaps sceneries very frequently (some of the levels even feel quite a bit similar to the first Unreal game). Moreover, the enemies are also rotated between the levels quite actively. So, on this level, you fight these guys. And on that level - those and
those guys (and you haven't seen
those guys, for, like, 5 or 8 levels!). And it's just really well done in terms of variety. Oh, and there is no background music during the gameplay now, only the ambient noise.
Level-design-wise, those square intersections of dozens of extremely narrow maintenance tunnels, that were quite frequent in M1, also got ditched - and the majority of maps are pretty much easily navigateable without a mini-map.
Comparing level-design of two games in general... Well, the first game for me is a game of two (or rather two-and-a-half) really special levels, which served as natural culminations for it. The first of those special levels for me was G4 Sunbathing - and the other one-and-a-half you haven't seen (so there's no point in elaborating them). And everything other than that for me was kind of meh. In contrast to that, the second Marathon kinda doesn't have any particular highlights. It's really varied and entertaining and the levels are really different from each other - BUT there aren't any levels you can point your finger on and say "OK, this, right here, in one way or another, is a culmination to the game". It just kinda flows one into another naturally and keep you interested, and
in general, M2's level-design is really
much more varied and entertaining, but there just aren't any G4 Sunbathings, if you catch my meaning. Also, one other thing that is quite telling is that one of M2's relative highlight levels is memorable for the sole reason of being a (smart) throwback to one of M1's highlight levels, putting a different spin on it.
Another thing is that, in contrast to M1 (which started really damn hardcore - on the max difficulty, the beginning segment between the starting point and the first save terminal was one of the hardest portions of the game for me), M2 starts kinda relaxed, casual and "nice". However, don't count on it being that way throughout the whole game: it gains momentum in just a couple of levels, and its most difficult moments are easily as infuriating as those from the first game (remember that "Bob-B-Q" level, the one that follows shortly after "G4 Sunbathing", when Leela drops you into a fucking killzone? well, there will be a VERY similar level here as well), and the traps from the first game are also VERY much intact.
What else... Well, there is a significant addition here gamedesign-wise. Or rather a switch. There aren't any vacuum levels, but there are liquids now. And yes, you do need oxygen for breathing in liquids. And you can't use anything except for your fists there, which, as you can probably imagine, makes the liquids quite problematic. Moreover, sometimes the water level is set in such a way, so that it constantly "flickers" "now you are above liquid, now you are under it"-style, which gets really damn infuriating really damn quick. Oh, and keep in mind that the water surface intercepts all the bullets and detonates any explosives, so don't fire the assault rifle's grenade launcher in chest-high water. As you can see, the whole "liquid" thing is not very well thought out, and it can really get in a way sometimes, but it still does sort of work for the overall "variety" thing.
And, about puzzles, there are some attempts to introduce them here and there, but there isn't anything even remotely as terribad as that "Colony Ship for Sale, Cheap" you were talking about earlier (of course I remember that monstrosity!).
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Now, as to whether you should start this game, not having completed the first Marathon. The answer is "Absolutely NOT". Marathon 2 is a direct sequel, continuing the story threads from the first game and putting a nicely rounded (even if somewhat drastic) resolution upon them (also, keep in mind, that M2's resolution was supposed to be the canon ending to the series - and that the idea of Marathon Infinity, as a sort of "add-on" only came later, when Bungie decided they needed something to bundle with Marathon's level editor they wanted to release). You should, at the very least, read all the terminal messages from the first game: you can find all of them
HERE. Moreover, in order to appreciate that "throwback moment" I was talking about earlier, you'll need to actually watch a playthrough of a couple of Marathon's levels.
I can actually recommend
this let's play, which was really damn entertaining for me even considering that I've already beaten the game by the time I watched it. You'll need to watch the chunk starting with "Neither High, Nor Low" - and ending with "Ain't Got Time Pfhor This" (you can skip "No Artificial Colors" and "Beware of Low-Flying Defense Drones"), which would take a bit less than 2 hours of your time. Keep in mind that the guy was absolutly abhorring the game by that moment - and also keep in mind the he was playing AlephOne version with (hnnnnnnnng!) ugly filtering and absolutely botched scripting (a couple of levels from the aforementioned chunk, while being at least somewhat adequate in the original Mac Marathon, are absolutely fucking broken in his version).
Is it worth it to try to finish the first Marathon yourself? I'd say it really-really isn't. There is half of the game still left - and believe me, if you played on one of the higher difficulties, it still has more than enough in it to drink a couple more liters of your blood. Marathon is just a game with a problematic and a borderline broken core - and not a very carefully done one either. Although it does have at least a couple of its moments.
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As for Marathon 2: Durandal, it's also a problematic and, at times, borderline broken game with rotten core - but it's a game in which it's evident that they really-really-
REALLY tried, which makes it interesting at least from the artistic/design standpoints. Keep in mind, that the most famous aspect of Marathon series - the story - is indeed quite a bit better here, compared to the previous game (and yes, I'd say, the FPS genre at the time of M2's release couldn't offer anything comparable to that anyway). However, it's still evident, that the story was written by an amateur writer (also, both M2 and M:Infinity scripts start with slight retcons to the endings of previous games), and some aspects of its delivery are sometimes quite questionable. However, the ending and resolution (despite how bold it was) were really enjoyable for me - and overall, I'd say, I'll always remember this game with at least some warmth. Even though its core is almost as rotten as it was in the first game.
Win95 official port is absolutely fine if you can make it run on whatever OS you are using (XP works fine, dunno about later systems) - and if you'll use AlephOne port, FOR THE LOVE OF GOD, please, don't use retextures, texture filtering (set the renderer to "Software") and transparent liquids. You'll just butcher the graphics (which are really quite nice in this game).
Also, I really, really advise you to play Pathways into Darkness when you have the possibility. It's really a MUCH better game, than Marathon 1, in almost every respect (other than level geometry and engine sophistication), and it's just unbelievable to me, that such a game was made by a team of just 3 people (with the majority of work done by just one) and released half a year before DooM 1. It's one of the best FPS games I've ever played, period - and I simply don't understand, why in the name of fucking hell (well, other than "nobody played the fucking thing") it isn't remember as one of the timeless staples of FPS genre and as one of the breakthroughs in FPS design (alongside the aforementioned DooM - which, BTW, kinda pales in comparison on a lot of aspects). It's a silky smooth (and with true 640x480 support) and perfectly playable game with perfect responsive controls, highly unusual AND enjoyable combat (constant maneuvering and evading enemy projectiles in the web of intertwining narrow pathways), tasty dungeoncrawling elements, gameplay shifting and changing with almost every level while being connected to the storyline at the same time, and gamedesigning decisions which are really transparent and just follow naturally from the fantastical situation that the game attempts to simulate. In some sense, it's kind of "Ultima Underworld Shooter" with all the shooting-related parts extremely well done. It doesn't have all the DooM's awesome sound design and music (or any kind of music at all), and its engine is closer to Wolf3D than to DooM (with its lightmaps and Z-axis), but it really, really starts getting to you after a couple of levels. Seeing Marathon after that game was extremely disheartening - and I still simply don't quite understand just how the hell Bungie managed to fall the way they did between these two games.
If you decide to play that game one day, I really advise you to go for the original 1993 version of the game, the one WITHOUT textured floors and ceilings. They make the game look waaay too cramped and claustrophobic - and this game is tense and stressful enough as it is, without a constant feeling-of-being-literally-trapped-in-a-cage getting on your nerves. Not to mention that 1993 version IS the version made by a team of 3 people - and 1995 version is simply kind of not.
