I've played Fallout 1 and Fallout 3. Fallout 3 had overall more enjoyable gameplay and Fallout 1 nailed the atmosphere of nuclear wasteland, but Fallout 3 was close in that regard.
I enjoyed Fallout 3 more, but Fallout 1 had better roleplaying aspects, like, your skills matter in EVERYTHING and all your choices matter. in fallout 3, if you nuke Megaton, you'll be lucky if someone mentions it to you.
So, in future fallout games:
-Make moral choices (acttually) matter
-Make stats matter
-Bring back the reputation system
-Better AI
-Better graphics, physics and larger map.