I agree....having a simple per-thread vote up/down is fine for certain kinds of forums, of course.

You can also implement a vote up/down mechanism within each thread so that the most helpful responses bubble to the top. The downside is that it can break thread continuity, but surprisingly not as badly as you'd expect - the conversational stuff remains unvoted (and is listed sequentially as normal), but the informative answers get boosted to page 1|2|3