I've heard that comments can dilute keyword density and hurt your rankings, but this doesn't seem to be a commonly discussed topic. Anyone know if this is indeed the case? I don't see why it wouldn't be, unless Google has a way of separating comments from content... Note that I'm NOT referring to the dofollow/nofollow argument. I'm talking about keyword density here, not link juice.
It definitely can... you can spend hours optimizing your page to be about your keywords, but if you have a lot of long off-topic comments, your density will drop like a rock. Some things you can do: Close comments after a period of time - not only does this limit the total # of comments, it cuts back on comment spam, since many spammers are searching for older posts with some page rank attached. Have a separate comments page - under the post you have the comment form, and a link to the other comments, but do not display them on the post itself - then noindex all the comment pages. Split comments across multiple pages - only show a few recent comments and have a link to the rest of the comments.
It's only really a problem when you have 100+OBL (out bound links) as comments on your article/post, but if they are legit comments I wouldn't worry about it, and if they are all just spam you should just moderate that anyway, regardless of the SE's, as other visitors don't want to be a part of a blog that isn't taken care of.
I think it's a good question. However, if the comments are good, they should, at least some of them, include your keywords in it as well. At least enough to keep your keyword density between 4% and 7%, which is most commonly suggested.