fallout NV is infinitely more replayable. ive done all the guilds, every side quests i could find, all but one of the daedric quests cause i cant find the last one and both sides of the civil war and the main quest. there is literally nothing in skyrim that beacons me to come back other than mods and future DLC. with fallout 3 and NV im always wanting to go back and do more things and try different ways of doing stuff. ive done all the questlines in that game but im still not bored of NV and im going to pop in fallout 3 as well since you can now play them both at the same time with the same character with the requiem mod. :drool:
This is also an issue I think comes down to a lack of dialogue/interactions/options with npcs. Fallout 3/NV, a lot of conversations/quests can end in vastly different ways depending on what skills/reputation/stats/perks (yes, all of them) your character has, and then what choices you make. Every play through is going to be different.
Skyrim, npcs have the one conversation that always ends the same way. Quests have one outcome. You can make a different character, but he will always say the same things.