Did he seriously just call Slipknot death metal? T_T
If you really want to learn about death metal go here
I think MadassAlex pretty much got everything down. (True) Death Metal is just an extremely technical genre, and what he said about them being masters of their instruments is exactly true (e.g. Krisiun, Severed Savior, Gorguts). Also, it requires a lot of stamina to play a majority of the stuff out there.
At the Gates
They pretty much say it all for me. If you don't understand the appeal of at least one of them, you aren't going to understand death metal. Death metal features some of the most technically proficient musicians to ever live, some amazing lyrics (as in, the one's that don't sing about gorging pig guts...that's not exactly impressive lyricism), a very diverse vocal style (High pitched growls that have more in common with black metal, deep gutural growling, aka cookie monster vocals, and even clean singing), and some pretty unique song structures.