New For Legion:
Level cap increase to 110
I have to admit, seeing level 100 characters was strange enough when I first logged in, with their hundreds of thousands of hit points… Now that I'm level 110, and my Rogue has a cool million and a half hit points, I'm almost interested in hopping back into old raids and seeing just how long it'd take bosses to make a dent in my life bar.
Demon Hunters
It's taken a few expansions, but we finally have our second Hero Class. Unlike the Death Knight, however, Demon Hunters begin at level 98, and end their introductory zone at level 100 - ready to jump into all of the latest content. Do you think Illidan is the bee's knees? Well, it's time to get your cosplay on.
Demon hunters can specialize as a melee DPS or take on a Tanking role. They're also the only class who can double jump, which has already led to some interesting glitch/out of bounds showcase videos. I haven't grouped with a tanking Demon Hunter yet, but as a melee DPS, they are extremely mobile and capable of doing great single-target and burst DPS. Their AOE abilities are tied to cooldowns and suffer in that regard, but overall they seem fairly versatile.
It should be noted that only Elven races can be Demon Hunters. Silly lore, getting my way.
Complete Overhauls to Talents/Specs
Talents and specializations have received a pretty substantial overhaul - once you choose a specialization, your class will ONLY be able to use abilities specific to your spec. This has a couple of effects: first, it really makes your specialization feel unique within the class. An Outlaw rogue plays completely differently than a Subtlety one, and they share almost no abilities. Unfortunately, it also silos character specs into definite roles. You may have chosen a hybrid class (Like Paladins, who can be a Tank, DPS, or Healer), but you'll only focus on one role at a time rather than being able to mix and match from several trees.
Talents are now granted once every fifteen levels, allowing you to choose one of 3 upgrades for that tier. You can now change these at will in any Sanctuary (or via a consumable item crafted by players with the Inscription trade). The consumable item means you'll probably see people switching their talents around in raids to best match the mechanics of each boss. Some talents are obviously geared towards being more self-sufficient, and others only shine in group content, but you're not locked into your choice at all.
Oh, and for the avid PVPer, once you hit level 110, you can start earning ‘Honor' talents, which grant bonuses in PVP content. If you max these out, you can prestige and start over, thus tying WoW mechanics to Call of Duty.