If the topic is unique, I usually reply. If I can relate to or have experience with a certain topic, I tend to reply with longer, more thoughtful responses. A more lighthearted thread environment also makes me more likely to take part... seriousness is for Stingy Sallys.
Basically, if I think it's neat and haven't seen it twelve hundred billion times before, I'll reply... this thread on reasons for replying to threads is a perfect example, very neat.
edit* I realized that I am much more likely to reply to threads that focus on general concepts and abstract ideas rather than specific events and people... maybe the way that those events and people are much like other events and people, but nothing about particular people or things exclusively. Of course, this gives me the opportunity to say anything that I like without any obligation or commitment to it.. weasel words galore... but I also find the abstract ideas to truly be more interesting, which leads me to learn more about it, which gives me something to say, which helps me to end up replying