Warhammer stands out in one department more than any other MMO I have ever played... ...it inspires coope
On the mechanics end of the spectrum, nothing stands out drastically.
What DOES stand out in warhammer, is player cooperation.
And, the game itself deserves alot of praise for this. In games like AoC, which almost feels like a single player RPG at times, and Warcraft, where the only coop gameplay is raids that brings out the most mercenary of self serving players...many aspects of Warhammer encourage cooperative gameplay to suceed, and do it in very innovative ways.
The public quests, which many before have talked about, are the biggest contribution to this. since good loot can be had from this, it often encourages people to team up for the objective in front of them. Notice I said "encourages" rather than "force" players to group up. Groups of players can often keep eachother alive better, and achieve better contribution scores toward the event than solo players, contributing to a better bonus being added to their final loot rolls. Sure, single players can handle things just fine, and even in their case, they tend to play cooperatively, watch out for other players, and help keep them alive.
Also, on RvR servers, there are many areas where quests occur in contested zones, and taking over these zones makes solo questing easier. This also encourages players to team up, beat the hell out of the enemy, heal their teammates, and play together to achieve a common objective.
Lastly, helping out other players helps your personal renown score, which also opens up more gear to be had.
Now, I know I've cited alot of self serving reasons for this cooperation, but since almost every aspect of the game involves cooperative play, it affects the game world in a very positive way...
...often traveling solo, players get random help from strangers if they get in trouble. And, sure, ganking is still a fun way for asshats with confidence issues to pass the time they have far too much of, but if other players are around, expect some awesome and extended world PvP to ensue from these scenerios.
What I'm getting at is this...people play MMO's to PLAY WITH OTHER PEOPLE. And no game I have ever played has encouraged people to play together more than Warhammer. Some of these part time allegiances are merely fun ways to pass the time, others form lasting bonds with some very good people. But, make no mistake, it is Warhammer's execution and careful attention to event and world participation that encourages this cooperative and public gameplay, which generates a more positive and helpful attitude from the player base at large than most other games.
And, these steps to encourage cooperation makes the game fun, makes the time playing with other people rewarding rather than a begrudged experience in a self serving attempt to get loot, and get people playing together from step one, which ensures that people will play together for a long time to come.