AtomicGamer interviewed Diablo III Lead Content Designer Kevin Martens. He mentions some of the games with RPG elements that have surfaced over the decade, but also points out gamers continue to purchase Diablo II copies eleven years later.
Things to expect from Diablo III:
The Runes system further diversifies each class. Example: The Witch Doctor has a skill named Plague of Toads. Customizing this ability with runes will allow the Witch Doctor to transform into a giant toad to swallow enemies. He will spit out the gold and items afterwards.
New areas of Sanctuary never seen before.
Better Quest System with deeper stories makes it more replayable than Diablo II.
Co-op Multiplayer better than ever.
You don't need to buy a new computer to play Diablo III. Low system specs covered.
Randomization: You might find the same monster in a dungeon, but it might have different powers than the previous time you played there. Sometimes it might be a different monster. Or a different quest each time you visit a dungeon.
Diablo III has been built from the ground up with co-op in mind.
A couple of extra things this interview mentions is the possibility of Diablo III for consoles. Martens says: "We’re definitely looking into it. We’re seriously considering it. I would say that the whole system specs thing is one of the cool things about Blizzard."
Blizzard Entertainment has been looking for Console developers for the past few months to explore how well Diablo III meshes up with console interfaces and controls. Martens also confirms plans continue to deliver Diablo III to stores in 2011 -- however, no official release date has been provided, and as with all Blizzard games Diablo III will be out when it's ready.