The comment policy on most of my blogs is: use a real name (first and last names), and if you include a link to a web site, the web site shouldn’t be pure advertising drivel. Something like “Jones Travel Blog” or “Something Tips and Tricks” would not be approved, as those sites are just wanting link juice. And I don’t use nofollow.
Jesper Rønn-Jensen at Time To Revise Our Comment Policy came up with five categories of comments. His categories of A, B, and E would be marked as spam on my sites. Categories C and D don’t matter to me, as I don’t count the gravatar as a positive or a negative, since the commenter can change their gravatar at any time.