I love the political and engaging threads. And pony threads with me a good thread is one that gets you into it and the thread it self have twists and turns. But stays MOSTLY on topic and doesn't spiral into shit.
I guess the biggest thing is just that it's active. I love a good thread where there is some discussion going on and sharing of ideas, but the biggest thing is just that there are plenty of people to keep it going. It could be a meh thread with a relatively uninteresting premise as long as there are plenty of people there to post at and talk to and such. Also cute ponies. I hang out on /mlp/ a lot because that's where you will find all the threads dedicated to cute little ponies and cute little scenarios and such.