Reaper of Souls centers around Malthael, the angel of death, who has seized control of the black soulstone, which houses the soul of Diablo himself. Blizzard promises a "darker" game this time a round, and Malthael is certainly dark, able to siphon souls from the people of Sanctuary, even allowing him to turn citizens into his minions. Of course, Reaper of Souls will include new monsters, including fallen angels.
The Crusader class is a holy knight, not unlike Diablo 2's Paladin. They are described as "a war machine made human. The Crusader packs skills like shield bash and a deadly technique called Falling Sword, which basically turns the Crusader into a bolt of lightning crashing down from the heavens.
The loot system is being overhauled, creating what Blizzard is calling Loot 2.0. This new system includes "smart drop," which makes it more likely that items you pick up will be tailored to your current class. The amount of loot will also be reduced overall, keeping players from acquiring masses of useless items. Along with the new loot system is a new artisan, the Mystic. She allows players to re-roll affixed stats on items, giving them a chance to create an overall better item.
The headlining endgame option is being called "Loot Runs," which places players in a fully randomized dungeon with randomized monsters. The idea, if you haven't guessed, is to slay lots of monsters and collect loads of loot quickly. The Paragon system is also getting some changes, notably that there is no longer any Paragon level cap at all.
Unfortunately, no release date was announced, so you'll have to wait to find out just when you'll be Crusading and Loot Running. Reaper of Souls is playable on the Gamescom show floor, and we'll be learning more about it before the week is out.