How Can You Tell If A Spinoff Series Will Succeed -- Or Fail?

Spinoffs can vary pretty wildly in quality, from shows that are good enough to stand all on their own to shows that make even their predecessors unwatchable by association. But what are some of the signs that help you figure out which kind you're getting?

After learning some details about the spinoff that was confirmed for Supernatural, a discussion began about just what that might look like — and what would determines whether a spinoff will work or not. And one key point was brought up:

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This isn't Buffy; Angel was a great spinoff because it took an established character and fleshed him out even more, with the backdrop of being in the same "universe" and having some familiar faces, while still having his own adventures that sometimes did and sometimes did not cross over. This, show, though...completely new protagonist, no history or depth - its like they're just using the Supernatural name but making a completely new show.

What makes a spinoff work for you? Tell us about some of the most shining successes and egregious failures — and what made them that way — in the comments now.