When you wait for an answer, it would be nice if a message will be popped and say: "John is typing an answer/comment" as in Google Talk.
Given how often I start typing an answer, only to change my mind halfway through, I don't see how this could possibly work.
Not interesting. What actually matters is real answer. Not people trying to answer.
Given that I've been often preceded by other poster who said the same thing I was writing, I guess I'm not the only one that did not submit an answer because someone was faster.
That mean too many notifications for thing that will not be concreted in an answer.
Don't bog down my connection with useless Ajax requests while I'm typing an answer, only to let the OP know I'm typing something. I'm pretty sure, statistically, the OP is not even on-line. It'd be just wasted bandwidth...