Nothing. Well the one I had for Hero was good, durable and functional...however they are overpriced.
The Evo, I am on the second, the first scratched my phone and was loose, the second was tightly on, but the casing has started to fade and scratch off within a month of use. I really don't abuse anything I own, the only reason I use is a case to keep a device like new so when I go to sell it someone else can enjoy it.
Not too impressed for a $25+ price tag. Honestly an eBay $10 would be just as functional IMO.
I just ordered the Case-Mate "Barely There", we'll see how that one is.