One major rule is no spam! None whatsoever.
A comment will be deleted if it's:
* Irrelevant to the post or site in general.
* Naked self promotion (if the comment has an external link and doesn't show that the commenter read the content).
* If it contains overly offensive language (I decided what that is). I don't care if someone says "omg help maya keeps telling me mr has aborted, this shit is driving me nuts.", but if you're dropping f-bombs every other word and they don't even help convey the point then that comment has got to go.
* Last but not least, if I feel that the comment is toxic, its gone.