Before WoW Blizzard had a new action rpg which, considering it is still a psuedo 2d isometric hack-em-up is still one of the best rpgs I've ever found (this for me is the PC equivalent of Zelda Ocarina of Time). With the expansion pack with the extra classes, an extra chapter and talents, characters level up as per normal but quest on randomly generated maps. Shedloads of items, loads of specs available and very quick action make this game huge.
Also upto last year new patches were still being released (Blizz are still supporting this even now) and can be added to your Battle.net account (co-op servers are still running and there is still an active playing community for it even now).
Blizz are famously still working on a sequel to this, but it's going to have to be something special to beat this. If you've never played it and like a good challenge, very few games even now come close to this classic.