A good story line I guess.. or setting.
If it's a decent setting the 'story' will write it'self. x3
But some prefer a set RP.. with some background to it.
It depends if you want to be able to control where the story goes somewhat.
Instead of just letting others make it up as they go.
I think both are fun.. but a background is more for serious RPs..
where fun ones all you need is a basic setting.. as you can see
from most of the RP's in that section.