I think it's directly linked to posters. Or voters on a poll.
Because views just mean you've looked at it, perhaps because the title was interesting and/or misleading. But for people to post, they have to have something to say. Therefore a successful thread will be one with many posts. Or in Pokémon Manga / Comics, one that has more than 3 users reply to it in a month :P
However, if it's a question then it's successful as soon as a satisfactory answer is produced, whether that's one that's produced within the first couple of posts (i.e. a question about tech) or 8 pages later (comparing two different pokémon, for example).