abuse, flames, spam, swearing, posting inappropriate links.
shoutboxes are instant, and unmoderatable,
it's bad enough when a spammer comes and starts pushing their adverts onto the forum already, without the option for them to come and post them in a box that we can't remove comments from...
also, personally, I don't like shoutboxes, other than just allow disorganised conversation there is very little that they actually add to the forum,
I mean that whol point of a shoutbox is just for saying stuff like, "hello", "this site rocks/sucks/rules/blows", "happy christmas" etc...
if anyone wants to say that then they could always start a thread.