I'm speaking solely from a 1x1/possibly small group perspective, since I tend to find little luck in larger group RPs nowadays. 

For me, setting a good relationship with my future partner, and for the pair or group of us to be able to talk out and plot a good "forward direction" for the setting or plot, doesn't necessarily have to be in depth but certainly should allow us to decide together what kind of tone etc we want for our plot, whether we want to go gritty and dark or happy and cheerful or a mixture of both and so on.

Also good effort on the other persons part to help me keep the plot running. Nothing irks me or kills my enthusiasm for an RP more than a person who ignores what we previously discussed about a certain plot (particularly in fandom cases) to then post nothing but half-liners or one liners (I have no issue with short posts, don't get me wrong, AS LONG AS they give me something to respond to) that are internal brooding/monologuing that my character(s) can't even respond to.