Diablo III? You probably don't want to know; it's not going to be pretty. The game's first expansion, Reaper of Souls, was announced today at Gamescom, and the fact that Malthael the angel possesses the soulstone harboring Diablo himself is all the justification you need for fighting him. This is a very bad thing.
All right, the mechanical side of things does come into play. Even if you don't care about the lore or about fighting off fallen angels, you can still care about features like a refined loot system that focuses on providing less loot that's more likely to be useful instead of quite so much garbage. There's also a new class being added, the Crusader, who looks similar at a glance to Diablo II's Paladin. Add in a new NPC who can reallocate stats on gear and you've probably got all of the incentive you need to look forward to the expansion.