Well, let's see.
-The selling point of the game was PvP, and if you don't know, the majority of online MMO gamers usually prefer PvE in which category WAR only offered a big fat goose egg.
-Warhammer was a very watered down version of WoW, all the depth and sophistication of WoW's RPG elements was completely missing in this game.
-There was so many RvR lakes that the populace of the server was always spread too thin, and since the games main feature was RvR, it was a giant disappointment.
-The classes were balanced around completely being dependent on each other, and here Mythic really screwed up because almost all gamers usually prefer being independent. In WoW no class is really dependent on any one else, some classes although do better on their own. In WAR it was completely paper/rock/scissors, if you were a Zealot there was no way in hell you could beat a Witchhunter, unlike a shadow priest vs a rogue in WoW, there for you always had to run in a group.
-Once you found RvR, every battle turned into a Zerg, I mean as awesome as 40 vs 40 might sounds on paper, it all just turns into a clusterfuck and after 10 mins you end up preferring small skirmishes instead of being stun locked and gang raped by 20 Marauders all at once...0o