Honestly, I've recently stepped away from Diablo hoping that RoS will bring the needed changes to make Diablo amazing again. Personally, I'm really looking forward to Game Modes, the new class, and loot 2.0. If I had to pick one... I'd definitely say the new class.
I'm really excited for loot 2.0, and I was really excited about some of the class changes. I play a Demon hunter myself and can't wait to level him up.
I'm pretty excited about the changes to current classes - I'm really hopeful that it opens a lot of other options for playing at end game rather than pigeonholing players into once specific set up on skills. WW Barb is getting pretty old, I like a lot of the other skills they have to offer, but they just don't seem as viable.
The RoS datamined bosses and creature models are what got me the most excited, wonder exactly how the encounters will play out. Adria sure looks creepy as hell (pun intended).
The class updates and changes or my favorite bit of RoS information! Although there is so much awesome new content to get excited about! It's all good!
For me the Class changes and COW LEVEL are the most eye catching of the data mining, but that is not all that is going to be interesting to see. I just hope I am still happy with the actual live changes.
The most exciting info I read was the datamined info about the loot properties. I know they are all subject to chage but, it still looks amazing. I get giddy like a little kid every time I think about loot 2.0. =)
-Yoda1337