The Order faction gets the Knight of the Blazing Sun, a defensively-oriented tank for the Empire army, which had thus far lacked a tank class. Meanwhile, Destruction players will have access to the Black Guard, another tank class for the Dark Elves, another race that had until now been without a tanking option. The Knight of the Blazing Sun specializes in supporting allies with group commands, while the Black Guard has a more destructive bent, wielding giant halberds and soaking up damage to build offensive power.
The new classes are actually old. Both were originally planned to make it into WAR's initial launch, but were scrapped because Mythic thought they weren't up to snuff and "would be part of WAR only when they were great and deserved their place alongside all of WAR's other compelling classes". Mythic promised they'd be included for free and not as part of an expansion, and today has made good on that.
New content has also been added in the form of dungeons and quests, while armor and items see significant tweaks and balancing. Other minor additions include dynamic player statues and a couple things that World of Warcraft players may take for granted: item hyperlinking in chat and support for add-ons. WAR's somewhat bulky UI might just get a little bit more elegant to use.
With the addition of the new classes and the added content, one is tempted to say that the Warhammer Online we should have received at launch is finally here. No question that there should be more than enough here to please current players. It's a lot harder to say whether it'll be enough to win back all those that strayed to that other MMOG.