Regarding FO 1&2. I would of liked them to have put more work into the sneak skill, and maybe just stealth in general.
Sneaking behind a lone NPC you didn't like, stabbing him in the back at night time, and not pissing off the whole town. Now that would of been cool.
More emphasis on survival and making the game harder, would of been fun too. Expanding the penalties of radiation, poison, hunger, thirst, and addiction.
Barter and gambling, were also too overpowered, even at semi-high skill levels.
Other than that, I can't find too many things to gripe about, when it comes to Fallout 1. The game may be a little on the short and small side. But it is HIGHLY replayable.
With Fallout 2, the game world is significantly larger, but for me, it has less replayability than Fallout 1. The Temple of Trials and the Arroyo dialogues, really pulls me out of the atmosphere. The quests in Fallout 2, also have a "chore" feeling to them, at times.