I don't think that any thread can truly be considered successful just by looking at the number of replies or view count alone, but it has to do with the quality of the thread and the promotion of discussion within it.
I think that while views are an interesting stat, they really don't say all that much about the thread. For example, the views don't take into account some of the random bots that are browsing the site, which is why you might sometimes click on a thread and see
something like this, despite it being a thread that may be locked or one that's from a few months back. If there were a way to filter out the views of non-members, it might be a better way of measuring a successful thread, but as it stands, it doesn't mean all that much.
Replies are a slightly better way to measure success, but again, there are many threads out there (like in PVP or even here, for example), where it's an easy to answer question, which means you'll see a higher rate of reply. But, many of these threads are "one and done," where a poster will make their reply, and never look at the thread again. When you start seeing threads with high post counts outside of one these types of situations though, that's generally an indicator of some success (either that, or an argument breaking out).
As for whether the definition of successful threads change across different forums, I guess it depends on how you look at it. Obviously, you'll get fewer views and replies in some of the less active forums, but if there's still quality discussion from a smaller group of people, I would say that a thread like that could be considered just as successful as any other.
So with all that said, I don't believe there's really any quantifiable way to deem if a thread is successful or not. However, I think that when you start seeing quality responses and a healthy interaction between the posters of any given thread, I would call it successful, even if it has a lot of views and replies or not.