I think it's a little hard to explain quite how they do it, but it's true that it there is a bit of genius about It's Always Sunny In Philadelphia. It manages to somehow use the kind of humor we normally associate with crude sexist/racist punching down humor without actually being sexist, racist, or punching down. It also does the Larry David style cringe/embarrassment humor without making us cringe. Sure they've probably gotten it wrong a few times, but overall it's a pretty neat trick.
Also, good shots of my neighborhood.