WoW - Massive, open world, thousands of people in-game at the same time (barring server population issues), etc. Questing is done generally solo, while dungeons and raids are scaled group content, requiring anywhere from 30min for a 5-man, to hours for a raid. Loot is shared amongst the group that completed the content, with pre-set loot tables on all bosses.
Diablo 3 - Randomized, closed world, at most 3 other players, other than yourself, in your world, at any given time (if you make your game public, it can be ANYONE, if not made public, only your friends can join you). Game's entirety can be completed solo, from the main storyline quests, to the side-dungeons and massive labyrinths. Content auto-adjusts it's difficulty (health and damage of monsters, even number of monsters) based on number of players in the game. Loot is individualized (though can be traded) to each party member, and is very heavily randomized to the point you won't be able to predict anything specific dropping.