Have you ever run into players that are content to run things on their own or with their friends, but get hung up on the fact that they are not allowed to do anything other than slice of life or bar RP without having to get someone's nod of approval?
There is another side of the fence on this. I've been among and RPed regularly with people across various MU*s/MOOs who are happy to run things for themselves and others, but have their enthusiasm and ideas snuffed out like a match in a wet napkin by staffers who have, plot-wise, stuffed them into a little box they only open when the ST/GM gets around to running something. (There are those players who don't seem to be able to rub two braincells together to come up with anything to do other than TS and bar RP and wait to be taken by the hand and lead down the garden path to be sure, thus facilitating the need for STs/GMs, so I'm not saying planned plots are a bad thing at all.)
I realize this is a little bit of a sidetrack to the main thrust of the argument, but I always thought the reward of roleplaying is fun. XP always seemed like something you did to gain levels and thus be powerful enough to kill others - fun for some, but generally not fun on a MUSH, especially when it takes more than thinking up a name and grinding levels to get a solid character.
Whenever I see XP systems at the door, I tend to assume that the goal of the game is to gain power/level to inflict my story decisions on others, as this is usually what happens.
I dunno. Perhaps that right there is the crux of the XP problem.