I'd have to echo pretty much all of what's been mentioned above. Some things that stand out to me, though:
Class mechanics. Everyone should be able to find at least one class (OK, probably at least half a dozen) that appeals to them. The specific mechanics used by the various classes vary widely, and are very innovative. The mechanics also encourage fun and effective game play. Let's take a look at healers, for example. There are 3 styles of healers in WAR, and each faction has one of each.
Goblin Shaman/High Elf Archmage - These are hybrid healers/nukers. They use a mechanic whereby each healing spell adds to a counter that builds up nuking ability, and vice versa. So when playing a Shaman I'd start out a fight by nuking several times in a row, and that would give me a bonus to my next healing spell. Depending on the spell, it would generally be a lower cast time or a larger amount healed. So, for example, cast 5 nuke spells, and then my big heal was instacast. Or if I need to be healing, then I'd throw in an instacast nuke every few heals to take advantage of that. It really encourages you to fulfill both your nuking and healing duties.
Chaos Zealot/Dwarf Rune Priest - These probably feel like the most effective healers to me, but they still deal pretty respectable damage when needed. They get a mix of heals, damage spells, and buffs/shields/support-type spells. Their buffs are cool, in that when a character is buffed, depending on which buff is placed on them, it actually gives them a clickable ability. For example, Icthus' Rune Priest buffed my Ironbreaker last night with a STR/INT buff, and while I was buffed I got an extra ability to deal 75 instacast damage to my target with a 60-90 second cooldown. Kind of a cool mechanic. They also get area buffs, that give a certain buff to all group mates within a certain radius of the buff. While they can deal damage, they probably don't deal as much as the other healer-types, but are very tough in RvR as they can heal through crazy damage and stay alive long enough for the cavalry to arrive and bail them out.
Dark Elf Disciple/Empire Warrior Priest - These are the melee healers. They use Action Points (which replaces WoW's mana bar for all classes) to power their melee attacks, and Soul Essence/Righteous Fury to power their heals. They start with a full pool of their healing points, and obviously each heal uses up some of these. The way they refill this pool is by attacking. Each melee attack refills a certain number of these points. So you can heal, but in order to keep healing, you need to be mixing it up in melee (although you can also covert AP to healing points in case no enemies are in melee range). Their heals tend to be very quick casting, as spell pushback would otherwise be a big problem, and most of their heals are HOTs or group heals. They also get several melee attacks that turn a percentage of their damage into various healing effects. They also both get other leech attacks (STR, AP, and such).
So you can see with just the example of healers, that one archetype has several different mechanics that play very differently.
Class effectiveness. There are no "support" classes in WAR, but virtually every class has some support abilities. Most of us have played a character class at some point that, while necessary for group success, wasn't terribly fun to play. They don't have that class in WAR. I've played multiple classes of every archetype in beta, and they ALL have fun stuff to do. Everyone can deal damage, everyone can support their neighbor in one way or another. They don't all do each equally well, but if you find yourself standing around with nothing to do in battle, then you're doing it wrong.
RvR dynamics. Mythic has deliberately tuned RvR so that fights last more than 5 seconds. There's not going to be any Rogue popping stealth behind you, stun locking you, and dropping you before you can do anything. RvR fights, even 1 on 1, last 20-30 seconds. This means that you actually get a chance to use your skills and strategy. And nobody has an "I win" button. In fact, what stealth exists is extremely limited, so you won't find stealthed gank squads running around in the open RvR areas waiting for unsuspecting victims.
And while there is "crowd control", you won't find yourself losing control of your character. Can you be snared? Yes. Can you be feared all over the zone? Nope. Nobody has fun when they die without a chance to do anything, and that doesn't happen in WAR.
Consequently, tactics matter in RvR. Use terrain, use flanking maneuvers, use collision detection to block enemies from getting to your back row. Get creative, because it WILL make a difference.
The bottom line is that this game is insanely fun, and it's only getting more so as we're getting more people and running in guild groups. I tend to be more of a PvE player than PvP. I enjoy PvP, but usually it takes an effort for me to go seek it out. Not in this game. Especially after the last few nights. I think I'm going to have to force myself to step out of the RvR areas and go find PvE to do at times, just because running RvR with guildies is so much stinking fun. And this is even before we get to keeps!