I don't think the racist and similar ones are the worst. Usually people will just ignore them and they'll get downvoted to hell anyway. Example from Slashdot: "You forgot to mention that you're still gay."
I read it and forget it. It doesn't mean anything.
The worst are the flamebaters. Another example from Slashdot (context is Firefox compilation hitting memory limits):
A- My resource usage rarely goes about 1 GB with multiple applications open.
B- If your usage rarely goes above 1GB then you're not a user of Firefox
The problem with this is that it's poised to incite a discussion between the people who feel strongly about the issue without adding anything new or useful.
First there are the discourse enhancing trolls, those who provide value to a discussion by suggesting alternative, usually taboo points of view.
Then there's the troll trolls. You know the type. The ones who spew racist or derogatory comments just for the hell of it.
Many people commonly mix the two into a single bucket, which I think is wrong to do, since one is a ++ for a forum, the other a -------.
Anyway, that's my take on the situation as someone who has ran 3 mid size forums in the past.