I was sorely disappointed with a few things about Fallout3 / Fallout: New Vegas that didn't follow the spirit of the original Fallout / Fallout2: the decision depth.
In the original Fallout titles, you could be the biggest bastard in California, and the game would adapt to that. You could steal anything not bolted down and only have a problem if you got caught, murder random people by sneaking lit dynamite into their pockets or by spamming super-stims on them, pickpocket people and then sell their own shit back to them, etc.; you could still get on in the game, sometimes with hilarious consequences.
With Fallout3, they tagged 90% of everything with a karma reduction so sneak became completely useless. You would even lose karma if you took stuff out of a house belonging to someone that was dead, and not dead by your hand. If you decided to play an "evil" character, half the NPCs just shut down and wall off half the game from you, rather than dealing with you begrudgingly like the earlier games would. It was like a sanitized version of the Fallout universe that was vaguely related, and with none of the creativity.