For me, part of it is that I need to be in the thread almost immediately when it goes up. After a thread has been sitting for a while, there's not too much point posting, especially since you have to be extremely careful with what you say. That's mainly because things you say get dredged up again years later or snipped out of context. The other thing is to consider the value that I could add to a conversation. When it's primarily a complaint post, there's usually not much to add outside of "I acknowledge your post and read it".