Yep, Diablo is finally a viable Warrior for really! I've only been playing him for about an hour, and still need to do more testing now that I am actually interested in playing him over a pick like Chen.

Variations:

Level 1: Toss up between Devil's Due and Soul Feast. If you feel you need to be tankier, take Soul Feast.

Level 4: Amplified Healing obviously has synergy if you take Soul Feast at 1. In rare cases it may even be okay to pick Swallowing Flames. I need to do more testing with it against certain comps.

Level 7: Battle Momentum works well. Endless Death might be viable, but I really feel like Devil's Due is better than increasing maximum amount of souls for the purposes of tanking.

Level 13: Take Spell Shield if it's better suited for the opposing comp.