[Ipstenu (Mika Epstein)]

Sam’s first point, about logged in users … With wp-super-cache, if you’re logged in, you don’t get cached pages. So on a forum, most folk are logged in, thus defeating the purpose.

Single topics pages can be cached untill a new response comes in.

That would be new and different, and IIRC not a part of bog-standard WP-Super-Cache, so that’s a pretty hefty write.