Hmm.. To me its an opportunity to contribute to a story written by the rp community.
Like any good book, che characters speak about thing from THEIR perspective, not the author's.
... They are reasonable, balanced, believable.
... They interact with one another and with their environment.
To me a good rp should contain the same elements that you see in fiction:
An introduction or stage where characters meet one another, become known, congeal..
Some form of conflict at some point. The degree and type of conflict is a matter of personal preference.
Rising action, some twists and turns...
A climax - whether its a fight, revealation of new info, a tragig loss, whatever..
Some sort of ending
I'm not necessarily refering to a particular event when I talk about these elements. To me, they should exist over the lifetime of a character, and also with regard to shorter term episodes.