SHIT dude! WHAT a coincidence that you would bring this up. :D I was just watching some earlier this evening.
This and Adventure Time keep me interested thanks to the soft TV-PG rating (with the scary bits, deaths, cartoon-ish limbs being severed, use of "crap", "pissed," "junk", etc.) It's edgy enough to be interesting, but not worthy of Adult Swim TV-14 stuff. (It's pretty shallow, but I'm not interested in the TV-Y7 stuff; I already had that back in the day. ;) And stupid, but Rigby and Mordicai are likable enough to keep me coming back (and the other ones like Pops, Benson, and co. balance them out well).
It's almost enough for me to make a show similar to Adventure Time, Regular, MAD, etc. - those that have the soft TV-PG. :)