South Park is funny, and will remain funny for this reason: They can make the episodes fast enough that they can be topical to current events. No other show on T.V. can do that the way South Park can, and they do a super job of it.
Another thing South Park is good at is knowing when to use their inside jokes. They also know when to -not- use said jokes. When's the last time Kenny died? (The PSP episode, I know, but it isn't every episode now, is it?). Also, Timmy isn't in every episode declaring his name.
I'll watch South Park now, and I'll keep watching it untill Trey and Matt stop making episodes entirely.