- Hermetic magic that seems to me to feel more magical and arcane than most other magic systems I've seen.
- An ancient conspiracy that isn't a unified secret behind everything.
- Black magic.
There's lots and lots of stuff that makes me go 'Oooh'.
Right now, I'm leaning towards using GURPS Cabal's treatment of other planes as the way planes work in my D&D campaign. Partly because it's just different from the standard D&D model, but mostly because it's so much cooler than the D&D model. (D&D3 might have a better model, actually--my memory is based on first edition AD&D.)
I want to point out that even if I do use this model of the planes, this doesn't mean that the players in my game need to pick up Cabal--after all, the other planes are supposed to be obscure to all the characters.