Nostalgia's probably the primary reason for most of the hatred of Fallout 3, despite the reasons frequently cited (which are valid.) I played Fallout 1 and 2 a while ago (never finished 2) and found them to be decent games. I preferred Arcanum and Planescape: Torment over them, though.
After playing it for a bit, it reminds me of a cross between S.T.A.L.K.E.R. and Oblivion, with the original Fallout games and a bit of Bioshock thrown in. I'll scribble down a few thoughts I have about it. Note: I haven't completed the game yet.
+ They kept the character development and choices of Fallout much better than I expected Bethesda to do, given the lack of meaningful social interaction in their previous games. It's not to the level of the old games, but above the majority of RPGs out there.
+I really like their implementation of the SPECIAL system over their earlier games of continuously increasing stats with obtuse level-up schemes. Them sticking to an actual experience system really helps to avoid the insane level-grinding that people who want to make powerful characters in Morrowind and Oblivion are strongly encouraged to do.
+/- Karma's better than anything in Morrowind or Oblivion. But the negative Karma associated with stealing from almost anyone in the game makes any serious thief forced to be evil. If I recall correctly, Fallout 1 and 2 both had better implementation of Karma, though that is a subjective opinion.
+/- Having distinct, interesting characters is not something that Fallout ever did well, and Fallout 3 carries on the tradition.
-Perks could have been better done IMO; the list's a bit short for something gotten every level and many are uninspired. Not carrying over traits from Fallout 1 and 2 also was an unnecessary blow to character diversity.
- The environment's not all that diverse. It's just ruined building after ruined building, with rubble scattered everywhere. It also diverts from the vast wasteland iconic of Fallout, which isn't necessarily bad.
- The game's stability very much sucks in grand Bethesda tradition; it's rife with crashes and has a few gameplay bugs (very much in keeping the tradition set by Fallout 2, though.)
-The inventory management is very much a Bethesda staple; a great deal of your time picking up countless amounts of useless crap and juggling it to sell it to merchants who don't have enough money. Limited inventory space and no way to sort it only compounds the problem. An automatic application of the Barter skill helps though.
- Combat's clunky in the style of Oblivion, and VATS has some serious issues with cover. Queuing up a few shots and having the all hit the wall right next to you annoys.
- The extravagant gore gets old fast and slowing down with every execution doesn't help. You could at least turn it down in the old games.
- I haven't seen a great deal that money is good for in a way that directly helps you in combat, unlike the earlier games. You can scrounge up stuff that is better than what you can get in shops, usually. Ammo is handy, but you can normally buy all of it out of any store you encounter for tuppence.
-The followers are possessed with insipid AI: a tradition of the series. Bethesda got it right here, except that they occasionally block your exit from a building and run in the way of your weapons.
I doubt 3's worse than Fallout: Brotherhood of Steel, but it might be better than Tactics. Fallout 1 and 2 are still better than 3 in my opinion.