I'd say that "success" is getting a lot of people interested in the thread so that they'll post. A thread without posters is like... I dunno. Can't come up with a cheesy metaphor at the moment. But posts from a lot of different people is what I think would make a thread a success.
I think you make a thread like that by not making it complicated, but it can't be too simple either. There's a butter zone that will get people to reply because it won't require them to write a tl;dr essay (which is a turnoff for most people) but also isn't so simple that someone won't bother adding their opinion (which comes from seeing a topic one too many times). I think the butter zone is more toward the simple side though.