My personal Favorite change has to be tauren paladin. I already paid to transfer my paladin to horde. Ill suffer with being a blood elf for as long as i need to to get a tauren paladin. I have been a paladin and a balance druid since right after release, and I will be rerolling (ZOMG) a worgen boomkin!
I am a weel known altaholic, with 8-9 80s on various servers, so the changes to the old world excite me so much! Also... I get to use my proto in the old world. awwww yeah.
I think the new system is going to be way more fun to play, instead of dragging for 40-50 levels before you get to enjoy it. As a person who played boomkin before they had a special form, i gotta say i love this early development stuff! Also very excited about mastery. As a paladin, I am stoked for the introduction of the "Holy Power" system, I have long wished there was something to do on my paladin other than hit buttons as they lit up!
Love you long time, gimme beta key.
-T