A game where even PvE players will enjoy PvP. Best integration of the two styles I've seen yet.
The first thing to note is that PvP does not feel tacked-on or like an extra feature in WAR. There are no stats designed just for PvP, and while your tactics will of course change when you're fighting players instead of mobs, I haven't run into any skills yet that are useful in PvE that aren't also useful in PvP (even a tank's taunt is designed to have PvP utility). The gear you can get from PvP rewards works well in PvE and vice-versa. While PvP has its own ranking system, you also gain regular class experience from PvP battles, so frequent PvPing is just as viable a means of leveling up as constant questing.
PvP is also easy to get into in the course of PvE questing. If you want to jump into a scenario (the more structured form of PvP, where two sides face each other on an instanced map), you can queue up for a scenario tied to the zone you're in at any time. After a match ends you're put right back where you were in the game world when you entered the match - no NPCs to talk to, and no warping from your quest area back to a city. You can go right back to where you were questing while you re-queue and wait for a new match. There are also several world PvP objectives, including keeps that require larger groups and some organization to seize. There are quests from warcamps that will take you into PvP zones, either to scout objectives or to kill NPCs, so even on servers that aren't open PvP you'll run some risks if you want to complete all PvE quests.
The endgame of WAR is primarily centered around the campaign to capture the other faction's capital city. This takes the form of multiple zones that must be controlled, either through controlling PvP objectives or completing quests. Unlock the zones in the way and you open up access to assault the city's main keeps, which look like a pretty massive undertaking. Get through it all and capture the city's king and your side gets bonuses and access to some of the best gear in the game.
Finally, it's worth noting that there's collision detection between players in PvP, and the skills and maps are designed with that in mind. There are benefits to having tanks in front, and reasons to attack tanks. You can block choke points with tanks and keep healers and ranged DPS behind them, for example. That makes for a much more tactical experience than just zerging the other side and hoping your healers don't die too quickly.
On the regular PvE front, most quests are pretty standard fare. Kill so many mobs, talk to this NPC, put a head on a pike, etc. Item collection quests are a bit nicer than many games in that every kill of a quest mob will yield an item, so no frustration when the random drops don't favor you. The quests are pretty well-written, and objectives are usually clearly marked on your map in the form of red blobs that indicate the general area where you can find an NPC or quest mobs.
There are dungeons in the game but not as many as one might be used to from other MMOs. Instead there are numerous Public Quests to be found as you work your way through zones. PQs are areas where everyone nearby contributes to the overall quest completion - kill 50 wolves, break 20 barrels, etc. You gain experience according to your contribution whether you're grouped or not, as well as "influence points" that can be accumulated to earn item rewards for each area. The open party system makes it easy to join a group for these PQs (handy for healers, and for getting healed), and once the final objective is satisfied everyone who contributed gets a chance to select a loot reward (the chance is weighted based on your contribution, and on how many times you've been through the PQ lately without getting any loot). It's a nice idea, and it means that even when soloing you can wind up working with other players.
A final PvE feature to note are entries in the Tome of Knowledge. The Tome is a combination lorebook/quest log/achievement log. You unlock entries in a variety of ways, both through PvE and PvP, but for PvE I wanted to point out the Tome unlocks that reward exploration. Scattered through the game world are NPCs you can talk to for surprise Tome unlocks, and there are usable items in obscure corners of zones that do the same. Tome unlocks get you a blurb of lore in your book, but also gets you some experience. Some can also lead to unlocking "tactics", which are bonuses you can tie to a character (like doing extra damage against spiders - you can change active tactics, but only a limited number can be active at a time). It's a good system for rewarding the sort of players who like to explore every inch of a zone, not just the quest areas.
I won't go into a list of classes and races, but would be remiss if I didn't at least summarize. There are three races per faction, and each has three or four classes specific to that race. Classes are designed according to familiar archetypes (tank, healer, ranged DPS, etc.). The races on the Order side are "good" and the ones on Destruction are "evil". They play those to the hilt in the quests, though there are occasional quests that hint at some hypocrisy underneath the good facades (like the witch-hunts carried out by the "good" human race to root out Chaos). The "evil" races are actually pretty evil. So don't look for too many subtleties in the lore, but have fun with the over-the-top nature of some quests (like an early Chaos quest to find a wounded soldier, kill him because he's clearly too weak to live, and bring his sword back so someone else can use it). Each side also has a race that works a good bit of humor into their quests and skills (dwarves and greenskins).
Population imbalances are evident, time will tell if those will be corrected or if each server is just going to be stuck with one side being outnumbered by their enemies. Higher server population does mean more frequent scenarios and an easier time getting warbands together to siege keeps, so be aware of that when you choose where to play. I find that even medium population servers can get regular scenarios running, but the population tends to focus on a particular match-up (so the human scenario will run more frequently than the elf scenario or the dwarf/greenskin scenario, since that's where most people will queue up).
Like all MMOs we've seen so far, there are some bugs and balance issues that will need to be addressed. Unlike some recent MMO releases (*cough* AoC *cough*), Warhammer Online feels feature-complete and fairly polished. All the hyped features are there on release, so the devs will be free to work on bugs, balancing, and new content.
For my own style of play this is a great game. In WoW (to pick a popular example) I didn't have much to do with PvP because I preferred PvE, and most of the rewards from PvP were aimed at helping just PvP. I also hated that PvP was something I'd commit to instead of PvE while I was doing it, so leveling would be put on hold for a time. That's not the case in Warhammer, so I find myself enjoying PvP a whole lot more. I can do that and PvE interchangeably with little downtime between either, and neither feels like wasted time. Both contribute to your success in the other.