Question is, how good is WAR's group PvE? That will only be shown with time. So far, changes like the public quests, open parties have been quite positive. Oh and, one HUGE change over WoW - CLASS SPECIFIC LOOT! So you never get yet another +damage mail chest nobody wants. 
Definitely the single-player PvE line (quest chains etc) is very good - great content/design, wonderful lore, good progression.
The one last thing I didn't mention which is a HUGE thing for me, is the class mechanics. A lot of classes have "orthogonal" abilities, such as eg the goblin shaman, the more healing spells you cast, the better your damage spells get, and vice versa. Or the warrior priest, who can only recharge "mana" through doing melee abilities, which puts him firmly in the role of frontline healer. Similarly, the bright wizard/sorceress have a delicate balance between going with full turbos and possibly even killing themselves but pushing out enormous amounts of damage, to reigning it in and doing more moderate damage.
Classes as a result are a lot more dynamic, there is no "stand at the back and heal", or "get behind the mob and stab", most classes have more to play with than just that.
I have to say I wasn't very happy with the mechanic changes I read about in WotLK, but a friend of mine in the beta says they don't seem to have a detrimental effect.

Definitely the single-player PvE line (quest chains etc) is very good - great content/design, wonderful lore, good progression.
The one last thing I didn't mention which is a HUGE thing for me, is the class mechanics. A lot of classes have "orthogonal" abilities, such as eg the goblin shaman, the more healing spells you cast, the better your damage spells get, and vice versa. Or the warrior priest, who can only recharge "mana" through doing melee abilities, which puts him firmly in the role of frontline healer. Similarly, the bright wizard/sorceress have a delicate balance between going with full turbos and possibly even killing themselves but pushing out enormous amounts of damage, to reigning it in and doing more moderate damage.
Classes as a result are a lot more dynamic, there is no "stand at the back and heal", or "get behind the mob and stab", most classes have more to play with than just that.
I have to say I wasn't very happy with the mechanic changes I read about in WotLK, but a friend of mine in the beta says they don't seem to have a detrimental effect.