tl;dr: I don’t know what makes a forum attractive to others, and I suspect I’m an outlier, but here’s a few thoughts on the subject that may help you further understand motivations for joining a forum.
Based on discussion flow. I think you may be thinking in terms of question/answer and help forums, but I’m tending toward simple community forums.
I think the Q&A forums really should go away in favor of a knowledge exchange such as stack exchange, and I’ve only asked one question to a regular forum that I wasn’t already part of once in the last decade that I can recall. I still don’t see a swimming pool knowledge exchange, and salt water pools are still new enough that not everything is google-able.
But community forums typically build up around a shared interest. For instance a forum associated with a webcomic (http://forums.penny-arcade.com/), or an activity like prop-making (http://www.therpf.com/). These places do provide a lot of support for questions and answers, but the primary draw to them is the shared interest.
So, for me, I’m looking for forums where discussions are interesting. Interesting to me means it fulfills my curiosity and desire to learn, my desire to share knowledge, my need for approval, order, collecting, and power. Or at least these are the things (in no particular order) that I’ve identified motivate me to participate in a community.
It seems that you are approaching this more from a perspective of having a particular need fulfilled, and searching for a forum that fulfills it. It may be that one has a specific problem, such as being stuck on a level in a certain apparently frustrating game, or having problems with one’s vehicle, housing, or relationship, and one is seeking answers they can’t seem to get via searching. I don’t typically have that kind of need - It’s very rare that I can’t find what I’m looking for via google.
But it may be that the end result is the same. You look for a forum that fulfills a specific need - what makes you stick around?
More specifically, what makes you come back after the initial need is fulfilled?
For me, it’s whether the forum also fills other needs that I have. One of those, for most people, is the need to be wanted, so a good community might try to catch web surfers by providing a quick answer to first draw them in, then inviting them to participate further by asking them a question. But there are many other needs. Some forums have casual games that try to draw people in through trivial free time pleasures.
But for me it’s the people, the discussions, and whether I can satisfy my curiosity, share my
arrogance knowledge, and match my level of involvement and discussion.
I don’t participate in the penny arcade forums because there’s way too much to keep up on. I can’t just sip from the firehose, but I can’t spend hours a day on something that is a free time activity for me either. Same with other super-communities such as reddit. Even though they might really fulfill my curiosity, not only is there a time/resource factor, but I don’t stand out, and that’s something I’ve found I desire to do in the communities I participate in online.
So don’t spend too much time looking at my response as definitive, but keep in mind that most people are motivated by a few things, and for me it’s those motivations listed that bring me back again and again to a particular forum.