Something that promotes a lot of discussion, or is interesting to a bunch of people. A topic that you can finish talking about in 1 post isn't going to be a good topic 9/10 times. Sometimes you can post something and people just aren't interested, there's not much you can do beyond look at what you posted and take onboard "alright, people don't care much about this right now", but maybe they will in a few months or something. That doesn't mean keep smashing topics against the wall in some sort of cycle until they stick though.
For instance, a topic like "What's your favourite chocolate?" is good because everyone likes chocolate, and they all have opinions on what's the best, what's the worst, how times have changed, stuff like that.
It's about finding a balance between interesting enough that a lot of people want to participate and having enough room to discuss it at length.
Although what I've said depends on what you consider a "successful" forum thread.
It's also something Lord Raven has talked about a few times so maybe go and find his posts on the topic, he probably explained what I said much better than I have.