The things I am most excited about is the Nephalem runs and ladder. Ladder is what to me kept D2 running so long as it made items and what not relative again and was fun to try out different classes.
Definitely all the data mined info that I've found here. I've liked this place since the Diablo 3 beta news and contests. Thanks for doing the great work!
I personally try to avoid any spoiler posts regarding anything to do with the story, but that being said the two things I'm most excited about is loot 2.0 and the crusader class, the moment I saw the little vid where he performs falling sword, I was SOLD hehe. And the fact that on my first play through I will be guaranteed some legendary drops from the bosses will make it that much more epic!
As simple as it is, the basic class changes. They're doing drastic work and looking to greatly increase the build variety which has been sorely lacking so far, allowing for more fun off the wall, but effective, builds which can alone breathe life into a game.
I'm most excited about being able to modify items through the mystic and the removal of the AH, this would create a much better sense of item progression than what is currently available in D3 and encourage people to actually play the game to progress instead of playing the auction house.
Oh man, the new item passive effects on the rings are crazy! Gain a shield equal to the amount of gold picked up, chance to shatter a frozen enemy, reduced cooldown on kill, and taunt!!! The build options look to be greatly improved upon and is a theorycrafters dream.
The information about loot 2.0, loot runs, paragon 2.0 are what have me most excited about RoS. Plus the info on clans, ladders and such. Can't wait for this expansion.
But in all seriousness, I think I'm looking forward to loot 2.0 and class changes the most.