Although I like the concept of threads in gmail, in practice it seems to randomly split out threads where I would think it remain. I find it confusing and if I do, the typical user must too.
I wish there were more configuration options. Like having ALL conversations between 2 parties stay in one thread. Like allowing you to associate one person with multiple email addresses. Like saying that multiple email addresses with different parties are all in one thread. Like letting you check specific emails and remove or add them to a conversation/thread.