PvP: This is a big one folks so please read closely. World of Warcraft has PvP but it is not clearly woven into the very fabric of the game. In World of Warcraft you enter into battlegrounds (or arenas) to compete against the other faction to earn points to unlock gear (higher arena ranks) to be better at killing the opposing faction to... repeat.  It's a very "arcade"approach to PvP and very much 'removed' from the game itself. In Warhammer PvP is the game. All roads lead to the Realm vs Realm (RvR) gameplay. In Warhammer the entire world is the battlefield. As you level up you progress through 4 tiers (level 1-40 divided) you battle against the other realm for a goal. That's another difference between the games. WoW's PvP doesn't serve an overall purpose in the game. Warhammer's PvP does. You can't progress into the last tiers of RvR combat without enough victory points accrued from previous tiers. Need further clarification? I thought you might so here's an instructional video.

Explanation on PvP tiers in Warhammer Online straight from the Devs in Video Format.

In Warhammer Online you do have something to lose in PvP. Some people think that WAR is pointless PvP. How does losing your main city to the enemy sound? How about losing actual territory such as Keeps (castles) that were held by your guild? (See latter differences for more explanation). And quite possibly the greatest incentive to do well in the RvR is that you need do better than your opponents or they will get stronger. If your enemy is constantly killing you then they are going to have Victory Points, more land, better skills, cooler looking characters, and you're going to feel like a dirty little squig for sucking. In World of Warcraft no one cares how well your faction does. If you all suck in the battlegrounds it has no impact on the world around you.

Keeps, Siege Weapons, and Open World RvR: This is huuuge. Remember all the promises Blizzard gave about siege weapons and open world pvp objectives, yada yada yada? Warhammer Online is actually delivering. Before anyone claims WAR just took WoW's ideas brush up on DAOC. Paul and Josh explain it much better than I do. Watch the video then read on.

Your guild can actually have a castle in the real open world. You're not going to be forced into a battleground to PvP. Your castle's guards, flags, and decor will reflect not only your guild but your realm. You can use siege weapons! Catapults and Ballistas were available in DAOC and you can bet they're available in WAR. Break down doors and slay players defending the keeps or defend them and shoot arrows down on attackers as a Goblin Squig Herder if that's your class of choice. It's endless fun that never feels the same like a Battleground. But hey, if Battlegrounds are what you like then Warhammer has those too!

PvE: This is where I want to talk more about quests and PvE in general. In World of Warcraft most of the game is centered around PvE. In the leveling game it's questing and in the end-game it's raiding. Quests in WoW are standard. Kill this, get this, do this. Again I can't explain it better than Paul Barnett and Josh Drescher. Basically say goodbye to stupid riduclous quests that require you to kill 50 things you've already killed. Bears, Bears, Bears.

Quest Explanation Part 1 / Part 2 / Part 3