If you were to transfer the posts to the hep-th mailing list or something, it'd be very bad content, and vice-versa. As it stands, I imagine when people go to 4chan they tend to get exactly what they want.
I would also guess that the average 'smartness' level is a lot higher than you might think.
There are also entire threads started on seemingly legit topics that devolve into flamewars almost immediately plus a huge number of people who seem to just spam some meme everywhere regardless or relevance.
I'm quite surprised that the intelligent people there put up with all the other crap. I suppose it's possible there are a lot of intelligent people with the emotional maturity of a 10 year old.
I think this sums it up correctly.
It's not necessarily a maturity thing. When I visit 4chan it's usually for a cheap laugh, not to engage in serious discussion. The Internet's big enough for all flavors of discussion, and 4chan fills its niche. It need not be considered childish for doing so. I can enjoy a rational foreign policy debate and also cackle maniacally while running down grannies in GTA. Neither exclusively defines my maturity level.