Here's the thing, you never know which threads will take off and which won't. Some of the most interesting topics run for a page or two and die. Other seemingly inconspiquous threads run for days and days. And for the most part, the life of a thread depends directly on which way it gets derailed by the participants. I don't recall any 20 page threads that were still on topic by the time it was on page 3. Sometimes they weave in and out of being on topic, but not the majority of it.
So when you see a thread that's moving on past 10 pages, don't even worry about what the topic is, get in there... there's probably a good time going on in there!
