To (a) and (b) I would also add (c): The signal to noise ratio goes down, so it's more difficult to find and upvote the best comments. Moreover, when there already are a lot of comments - and this can happen very fast - new comments, even very good ones, have a very difficult time emerging, and I guess this creates a disincentive.

One solution to try could be to give more time "up there" to comments from commenters with a high karma (only considering the karma from comments, not that from stories, which IMHO is a completely different signal).