This is a bit of a killer. It's not that I don't think getting to more is better, or even that it just depends on the topic, just that it should be somewhere in the middle. Basically, regardless of the topic, it posts shouldn't be long-winded and boring, but there is also no reason not to put quality notes/comments in shorter posts.
I have to say I agree with what most members have posted, thus far. Post count really doesn't matter. I'm a member/staff member of nearly 20 tech-related forums, but I also post as little across all of them as a one-time member looking for a single answer. When I do post, I try to keep it focused on giving someone a quality response, without overloading them with a long one. It's actually become practice for me on several boards to provide the long answers, but at the top of the post is a short, to-the-point, answer. If the reader wants additional information, such as why it is the answer, then they are encouraged to read further.
Just my 99 cents. ^^'