In response to the OP: Good role players. Immersion. Storytelling.
That's it. A good, rooted, friendly group of immersive role players will keep me content in a game where I dislike the gameplay, backstory, or anything else. Played WoW for so many years, and when my group of role playin' buds disappeared, I just figured out then how disgusted I was at it mechanic wise, quit. No feature keeps me from it, in fact I think good role play keeps me from noticing features I'd normally scoff at. Good outweighs the bad, so to speak.
If only a game was focused for in character interactions. Responsive NPC's - not every time someone does something, mind you - a lively, well filled out city (no three houses for every 1,000 citizens, please) and general 'realism' to a fantasy or sci-fi setting. It doesn't need to be real, it just needs to feel immersive. That's what I'd like to see, and I know it'll probably never happen, c'est la vie.
For me, in the end, all I can hope for is a good community of solid role players who know how to weave a story worthy of novelization. Something that's juicy, but not a soap opera. Something worthy of actors portraying on the large screen. Maybe I'm just a bleeding heart and I should wrap a scarf around my turtleneck and begin doing off Broadway productions of my own plays, but it's really how I feel.
No amount of fancy or glittery features will dissuade or persuade me to go someplace unless an RP community is going to be rooted, or is rooted. I know for a fact that asking any developer to support role players won't really cut the mustard, so to say, but it's a dream that one day a team will go 'Those role players are dedicated. Let us pleeeease them.'
TL;DR: People make the RP. Circular support from player to player will help a community thrive in spite of unfriendly RP mechanics. Not 100% of the time, however.