Endless options for class building, a dev team that cares and constantly provides new material for the game. It's free to enjoy most of the content, but they deserve every penny you throw at them. Their upcoming 2.0 and mobile release only add to the awesomeness!

[Edit 1 / 1000+ hours logged]:
This is the greatest ARPG of all time, and it is not even close. After over a thousand hours in the game, I made my first attempt at a hardcore SSF character and completed the campaign. It was exhilarating, terrifying, and utterly refreshing. This game keeps on giving and the developers are at the cutting edge when it comes to designing and implementing on classes and endgame in a way that makes the game feel bottomless.