Notch says he's skipping 0.9 (which would otherwise have been released in the past week) because he doesn't want to try to squeeze in another real release he doesn't really know what he's doing.
- they're huge
- they fly
- they hoard treasure
- they live in huge caves
- they have scales stronger than diamond
maybe.
All depends whether he implements them properly, and sufficiently rarely, to make it exciting to go hunting for them, and an achievement to find/defeat one.