My top 3

Hello everyone this is my opinion based on what I have seen from videos and wikis. (all this is subject to error so be gentle)

1.  First and foremost character customization.   It doesn't limite you to racial restrictions instead allowing any race to be any class which give for more of a diverse community and not having to look at the same blood elf warlock all day in your group, even though its a different person.   Not to mention social gear will relieve the monotany of looking at the same skins all the time.  Also during character creation you are given a series of choices to make that impact your characters strengths and changes dialog during dynamic events.

2. Character Optimization: The equipment you use effects your skills, and is much more than an attribute stick, Each weapon has certain abilities for different circumstances, combining these abilities can cause addition effects, for example an elementalist can lay down a fire wall allowing anyone that fire through it to gain buring damage on those attacks.  This is going to force people to think out and plan their tactis, making no two play-styles the same.   Also every profession has 5 trait trees these act as passive enhancements for your attacks and abilities.

3.Difficulty:  I like for my experience to be challenging, when I first began WoW in vanilla I was not hardcore I enjoyed the game immensly but it took me nearly a year to hit 60, and once I had done that I was overwhelmed with my choices of dungeons, and spent many hours grinding away getting gear, trying my best to save enough money to get my mount (ghost wolf until BC lol) and then I got into raiding and saw the precision that occured and the 40man "no room for failure"encounters in which a single person could ruin *cough baron gheddon cough*, and then as each x-pac handed down it got progressively easier and easier which was the main reason I left the game, but I diverse back to my point The difficulty scales to the amount of players participating in open world dynamic events up to 100 players in some zones, while instanced dungeons are scaled for a certain level range.   The holy trinity has been kind of diluted in GW2 also, every player has a way to heal and put off damage for a certain time, of course there are professions which are intended to be front line fighters and made be a little tankier, and yes there are heals, but from what I have seen in videos there are few heals which can be considered "life saving".

those are the reasons you will see me on release date, peace out holmes