Reread the Planescape books. There's tons of interesting things in the Great Wheel. However, most of those things can simply be imported into a different cosmology.Forcing myself real hard to think of one, I just come up with the fact that the positive energy plain turns your into hyper Deadpool and will pop you like a zit if you heal too much.
But that brings us right back to 'immediately lethal' as the con for a big chunk of it.
The weird thing is that WotC had a perfectly good positive energy plane model in Serra's Realm and has NEVER used it for anything.
Also, the dependence on alignment is an instant non-starter for me.