so, i play a lot of quakelive, and in general am a bit of a id fanboy. i loved doom 2 and i was my main game for a couple years.
what i liked about doom/d2 and therefore did not like in doom3:
in doom/d2 all the mobs are already spawned; they are somewhere in the map waiting for you, and if you manage to find a way to spot them you can snipe them, or anyway manage to *not* be in their spawn spot when they spawn, so they don't necessarely take away all your armor / hp.
in doom3 you can't do that. they will spawn on your ass and hurt you because id decided they would. also that's why you need so many medikits/shards.
in d/d2 you can skip huge parts of the map, you just need to manage to. fly over a wall, RJ your way to higher ground, TP somewhere high then jump down .. all this in what is essentially a 2d game with some of the best 3d maps ever.
doom3 is one big corridor.
in d/d2 you will be faced with WAY more mobs that you should consider trying to take out. and if you want to try to take them all out, you will have a lot of strafing to do. which is what you learn to do very well, eventually.
in d3 it's all a f* corridor and mobs spawn right on yo'ass, so the whole "i will strafe u long time" thing is just worthless.
this is the most annoying thing of d3. i suspect id did this because otherwise older doom players would have just whizzed trough the game. but the map design of d3 is just pathetic.
doom/2 had seriously powerful weapons - the RL and BFG were devastating, even from miles away (and you did get plenty chances to use them from so far); nothing better than throwing a few rockets in a small hatch to a room full of imps, on the other side of the map, that you will probably visit in 10 minutes or more for how far it is, and kill them all beforehand. a rocket well spent.
what i also did not like of d3, and that somehow d2 did better:
in doom3 you get scripted events, such as monsters crashing through the wall, that are supposed to scare you; well, ok, some of it was scary (at least startling), but when a platform in d2 opens and out comes a totally ordinary Cyberdemon, believe me darling, i shit my pants :/
the whole lighting/darkness/flashlight thing was ok, i guess. not my cup of tea, but not horrible either. FEAR did it better. but it was ok, as a visual effect.
it also was way overused and annoying and gave me headaches. tbh the whole engine was quite amateurish (LOL, try to set some modern resolutions) for someone like id, the story is still the same, locations were not really special/memorable (i liked wolfenstein better), weapons were meh, mobs were poorly balanced .. it was a very forgettable game.
Prey (which had a horrible "no death" mechanic) was better, doing the same thing with more style.
in the end, the only bit i had a little fun in was the final boss, which was too easy; i wish they had given me a few big rooms with plenty of space to strafe/dodge in, big gunz, mounds of mobs trying to shoot me (no hitscan weapons pls), and just let me do my doom thang...but they didn't.
there's really no reason to rehash doom3. it was a mistake and it's better to just forget it ever existed.