Dont mind IM:s/chat as platform for game at all, but it places some restrictions to game as well. What I have found is that for game to work over IM:s, story needs to be able to go along in short steps as there is less continuity, or needs to be short to beging with. I have not too many hours i could ever dedicate to something in one single day, so I prefer to get the most of the time that I have. In IM:s can actually get something done instead of one post if even that so in that way much preferred.
And Interactive scenes in actually interactive environment are always good of course. Forums are the best place for when one needs long, detailed post and good history of what has happened in the story so far. So the more complex and in-depth the game, the better it becomes if it is played on forum instead of instant messenger or chat, at least for me. Even played some that mixed the two in my days, so many things are possible in that front.