I HATE cartoon musicals (unless it's something like the Boondocks where it's satire). I dunno why, but it's always been my biggest pet peeve. So unless you're playing a hip hop/rap song from the radio (or just a funny rap song), I want nothing to do with your intro. I came to watch birds and raccoons SPEAK damn it. Just give it to me straight.
Btw my favorite thing about Regular Show and Smiling Friend's intro is that it not only tells me what the vibe of the show is, it also tells me that the writers have a story to tell. That they aren't shaving off one minute just to save time.
Here's themes that get a pass:
