Better writing and NPC character development goes a long way w/ me - it makes it much more enjoyable to actually RP rather than just play through.
FO3 characters seems so two dimensional to me and the plot/writing was at times painful to read. Really, my only interest became seeing how many mods it would take to make the game so difficult that I couldn't play all the way through on very hard level w/o dying at least once. Yes, the vanilla very hard level was that easy - I had multiple vanilla play throughs where I never died. FWE helped fix that.
With FNV I'm eager to try new characters w/ different backstories, different alliances, and different stat combinations that better reflect that character's strengths and weaknesses. Plus I love the fact that you can't create a god character w/ maxed out stats across the board over time. I want to go up against enemies that can overpower me in a straight on fight at lvl 30, but w/ a little strategy I can take on and after a long battle still may lose but have fun doing so.