What gets me really annoyed are the "irrational" tactics. The "why do you have to be right" approach is one, generally favoured by spouses. The "you're not a woman/black/poor/Irish/Roma/gay/married/parent/Catholic so you don't understand" is another classic which gets used pretty damn often (you know who you are ;-) to say that your argument is invalid because of who you are, not bcos of what you're saying.
As far as going for the "weak points", surely a debate/discussion/argument/whatever is all about seeing how strong the other person's case is, and whether they can justify what they're saying. In that case, all weak points in the argument are fair game - a weak point in one apparently-unrelated area may roll up your whole reasoning. Ad hominems aren't though, and I think pointing out typos/grammar should be considered an ad hominem attack on message boards like this, especially since many ppl are not English speakers and/or are unfamiliar with typing.