Everything you could want in an ARPG, and quite frankly is the 2nd best one I have ever played (Diablo 2 is my #1)

Many classes and class combinations to choose from.
Multiple play styles for each class and class combination.
Hundreds of skills to choose from.
Hardcore mode.
Ascension system to assign skill points for buffs and special feats.
Multiplayer, extremely fun with friends.
Single player, extremely fun on your own.
Interesting and engaging story line, with multiple choices that affect game play.
Various challenge modes, such as Veteran mode, Elite and Ultimate.
Endless wave/tower defense mode with Crucible expansion.
Boss dungeons outside of the main story line.

This game doesn't hold your hand, you actually have to explore every nook and cranny to find all the shrines, hidden bosses, secret caves etc. I explore every inch, and it took me about 80 hours to complete the game with the expansion packs on Veteran mode. Lots of replay value with the different classes, story choices and crucible mode. I highly recommend this game, even at full price.