The Alpha: Elective Mode - it is a big deal for me so it goes to number 1. Its got something to do with giving people what they've been asking for for a very very very long time. I loooove control customization.
The Bravo: Skill system - playing the 'early' game will actually be fun for once. Don't know about the late game so again, nuff said.
The Charlie: Vi-o-lenssssssssss hehehehehehe
The Delta: Multiplayer made the game fun again. I shall expand. One of the things that makes co op games fun is what other people bring to the party. This game will be more than just 'who does more damage', there's actually some player skill involved. So multiplayer is boosted.
The Echo: Epicness, there is more epicness involved. This is part of both the skills system and flashy graphics. But at least its there at last. It was one of the things I thought were lacking in D2 and I'm glad to see more of it in D3. It progressive and I could comment more on it but I'm just happy that the flare is now in the players control too.
They're my top 5 positives.