Monsters

The monsters are various and clever such as Thousand Pounder. It is a unique monster. Skeletal shieldman protects skeleton archers with shields. Cannibal corpse climbs up to attack players. Grotesques give out lamprey.
Magnificent death effect

Diablo 3 has different death effect including mortal strike and monsters killing players. Death punishment is not that strict. Experience points will not lose but 10% durability loses. In HC, characters cannot retrieve life if they die.
Difficulty lvls
It has four lvls, common, nightmare, hell and inferno. Inferno is announced at GamesCom 2011. players wanna play next difficulty lvl have to reach to certain lvl. common (0), nightmare (30), hell (50), inferno (60). The higher the lvls are, the lvl-higher the monsters are. These monsters are more challenging. Besides, different lvls have different suits.
Items

Similar to items in Diablo 2. items have random drops and random attributes. New characteristics:
Runes are not set to arms and armors. Runewords are removed. Runes are used to strengthen skills.
Each item occupies 1-2.
Storage space is expanded to 350 space.
Shared character storage box.
Each player can only see their own drops.
Potion is changed and limitations are added, such as cool time and lvl limitations.
AH and real-money system.
NPC
So far only several NPCs are known.
Deckard Cain—he comes back, telling story to players.
Leah—female in CG.
Craftsman—jewelers, mystics and blacksmiths. Good assistants of players.
Tyrael—archangel.
World map

Tristram leoric highlands, Simash are in Diablo 3. Compared to undercity, outside scenes are less random, and terrains are more stable. Interactive environment is set including mortal traps, obstacles, and destructive scene elements.
Dungeons
The dungeons are random and increase the playability. Each time players enter, dungeons are different and outside scenes are stable.
Traffic system
Waypoints are important traffic system. Compared to Diablo 2, the waypoints are designed more intense.