The success of a thread depends on a number of factors - but many of those factors depend on the thread title and the first message. I think the most important rule is Be Specific. The thread title should be different from the titles of other threads on the same topic, so that people know right off why it's different from what has been said before - if we have six threads all titled "Accordions," that's a problem. The first message should both give and request information, so people know what you already know, and what you need to know.
If it is indeed a new aspect of a discussion topic, then it certainly is justifiable to start a new thread. If it's a continuation of an existing discussion, or particularly if it is an answer to something said in an existing thread, it usually makes no sense to start a new thread.
-Joe Offer-