Here's a nice little compilation of the information we have about the expansion thus far...

Important Info
Release Date: 2014!
Malthael is the primary antagonist: Returned from the void as the Reaper of Souls, with his wings turned skeletal. Malthael wants to end the Eternal Conflict by destroying all demons and he’s not fond of humans either. He can turn the dead against the living.


Assets
Opening Cinematic: http://www.youtube.com/watch?v=Cb7QJwQ58T0
Gameplay Teaser: http://www.youtube.com/watch?v=zGp5dkJdi0w
Loot 2.0 GameCom Presentation: www.youtube.com/watch?v=5dUnezvW4Pg
(Shaky Cam)Crusader Demo: http://www.youtube.com/watch?v=pX3xu37dU1w
Press Release: http://us.blizzard.com/en-us/company/press...tml?id=10697951
Mini-Site: http://www.diablo3.com/reaperofsouls


New Features
New Class: Crusader: Really seems like a new-age Paladin (see skills below)
Resource System: Wrath
Skill Categories: Primary, Secondary, Defensive, Utility, Laws, Conviction
Known Active Skills: Sweep Attack, Shield Glare, Judgement, Laws of Justice, Heaven's Fury, Consecration, Shield Bash, Fist of the Heavens, Blessed Shield, Blessed Hammer
Secondary Active Skills: Shield Bash, Fist of the Heavens, Sweep Attack, Blessed Shield, Blessed Hammer
Fist of the Heavens Skill Runes: Tempest of the Heavens, Well of Retribution, Lightning Rod, Reverberation, [unnamed]
Known Passive Skills: Indestructible, Wrathful, Wrecking Ball, Against All Odds, Finery, Heavenly Strength, Holy Cause, Vigilant, Long Arm of the Law, Stand Your Ground
Lore:
Driven by a centuries-long quest to cleanse the corruption blighting their beloved Zakarum faith, Crusaders are warriors of righteousness who have been hardened through relentless, brutal combat with the foul evils plaguing eastern Sanctuary. In addition to wearing immensely heavy armor and wielding a wide range of cruel and punishing weapons, this new playable class in Reaper of Souls uses battle magic to strengthen allies and weaken foes. A natural walking tank, the Crusader adds power and versatility to any party of adventurers.

New Area: Act V, Westmarch/Pandemonium Fortress
Known locations: The Wolf Gate, Moldering Waterway, Old Storehouse, Streets of Westmarch
Seamless interior/exterior location travel: Absolutely no load time between traveling zones, this is actually pretty unbelievable. Watch in the Shaky Cam video above.

Increased Level Cap: Level 70 is the new cap for non-Paragon Levels. All classes gain new skills and abilities.
Paragon Levels now shared account-wide: Also no PLvl cap
Paragon Points System: Can be distributed between STR/VIT/DEX/INT, Move Speed, Crit, etc, and can be respecced!
Game Modes: Loot Runs and Nephalem Trials, sounds like MF runs and infinite dungeon game modes! Loot runs are shorter dungeons, 15-20 minutes and completely randomized.

New loot
Multi-level Legendary Items
Smart Drops: Items that will drop with stats tailored to the class you are playing.
New Blacksmith and Jeweler item-crafting options
Mystic Returns: Enchanting allows you to reroll an affix on a Lgendary or Rare items as well as a Transmogrification system.
Puzzle Ring changes
Serpent's Sparker

New monsters
Seraph
Summoner of Death
Executioner
Death Maiden: Mini-boss

Randomized environments
New Quests