I have seen criticisms of both (1) starting new threads that deal with a topic already covered before, and (2) revitalizing old threads.
“It depends”.
Yeah, I know, real helpful. But the point is that there isn’t a Golden Rule for this. There are, however, a few points to take into consideration.
[ul][li]If the relevant thread you find happens to be a pre-vB thread, please leave it where it is. We all know how messed up the chronology can be after the conversion. Feel free to start a new one.[/li][li]If the relevant thread asnwers your question completely, don’t do anything at all, I suppose.[/li][li]If you feel your question is too far removed from the OP of the old thread, you’d probably better start a new one, possibly referring to the old thread to indicate that you did indeed search the archives.[/li][li]A little explanation, whether while bumping a thread or starting a new one, always helps. If anything, it avoids the questions like “Did you search the archives?”, or “Why are you bumping this old thread?”.[/ul]So, ultimately, it’s a judgment call. But if you explain your motives, you should be OK either way.[/li]
Hope this helps.
I guess it always depends on each individual case. Sometimes, a thread can get REALLY hijacked into a flirt-fest a la MPSIMS and IMHO (note: I’m not saying flirt-fests are bad). Those two forums are more casual… and it’s usually less-taboo to start a new thread. In addition, you can usually add a slightly different spin on threads.
However, in GQ and the Comments forums, it’s just really obnoxious to have three “How do you pack a parachute?” threads going at once.
It just depends on what forum you’re posting too… and how much common sense you use. 
Obviously, my answer is more suited to the “serious” fora then. Of course, a man like Libertarian rarely ventures into the Fuzzy Wuzzy World of MPSIMS and IMHO. 
Then I would presume that in Great Debates, we should start a new thread if either (1) the old thread is pages and pages long or (2) the old thread became hopelessly tangential. On the other hand, we should resurrect an old thread if (1) it isn’t very long or (2) our argument might give new insight to the discussion. Is that a reasonable rule of thumb?
Works for me, Libertarian.