I know I've restarted a number of discussions for things like chosing cams, questions on heads, etc. I usually always do a search though before hand, and sometimes I see similar threads or threads that were kinda hijacked either into my topic or from my topic, so that's when I start a new one.
I've also found threads that haven't been posted in for over FOUR YEARS - that's when I start new ones, because the parts people are discussing may not even exist anymore!
Jeff