A few other people here have noted that Warhammer's population is spread very thin.
A game like WoW will suffer from a low population server. However, it is not its undoing. A WoW server can thrive despite its low population. You can do raids on a low population server.
For Warhammer, low population ruins the game, and I'm not entirely positive that they realized this in the beginning. Everything in the game depends on there being a constant avalanche of people doing things. This way you have people constantly sieging keeps, constantly in scenarios (PvP arenas), constantly doing public quests, and you're constantly fighting. The game (as I'm sure many of you know) is very PvP centric. A large population ensures that you can log into the game, find the latest war zone, and then go start busting heads. The game was designed this way, and now they're actually encouraging this. I'm hoping this makes the game more like it was envisioned. If they can keep server populations high they'll do well.