Slander and insults are also allowed as long as they contain any kind of constructive opinion, info or something else substantial and aren't off-topic. For example, posting something like "you are dumb" will be deleted as it contains no meaningful content. However, if the post is somehwere along the lines of "You are dumb. This is wrong because this website/thread/etc. has explained it's not right", it's in most cases accepted.
Do you really mean slander? There could be legal implications, certainly there are slander (and libel for print and other media) laws here in UK, and I'm sure variations of the same in other countries. Completely agree (FWIW) that if a potentially contentious statement is made it must be backed up with evidence.
Cheers, Ngaio
As far as I understand most laws only have laws regarding the expression of slander as it can be censored as it isn't considered any kind of creative, scientific or otherwise constructive expression. Aside from that, all legal matters regarding slander fall on the person who expressed it, since it's his decision.
Edited the post to change it to a bit more mild meaning.