I like to think people in the know can all agree that New Vegas *BLEW* Fallout 3 out of the fucking water...
-Damage Threshold was a way superior damage model to Damage Resistance.
-Emphasis on faction standing rather than "karma."
-Hardcore mode (If it doesn't make a return in some form in Fallout 4 if Bethesda makes it, then I would be wont to say that RPGCodex has a point. And believe me, that would be a terrifying thing to say).
-Superior and sensible crafting (It's Hand-loaded .45-70 SWC or bust for my Ranger Sequoia).
Seriously, I could never go back to Fallout 3 after playing New Vegas.