Tobold ponders "Will Order or Chaos prevail in WAR?". This has been my big question about Warhammer - how do you reconcile a system where the side with more people wins with real lasting consequences? (Warhammer promises to allow the sacking of capitol cities, which can leave the losing side without auction houses or bankers as a penalty even after they suffer the indignity of being bailed out by NPC's.)
Personally, I play on a PVE server in WoW because I don't find anything especially enjoyable about being killed while questing, not being able to enter instances because one side is occupying the portal, etc. (Apparently, a non-zero number of people who rolled on a PVP server, thinking it would make them more hardcore, don't like these things either.) I suppose spirit shards are actually a decent bonus, the [Mark of Defiance] is a great trinket for casters (though your side need not WIN to get the tokens for it), and it's good to have access to the vendors in Halaa once every so often when you have stuff to turn in. In general, though, WoW's world PVP is not about lasting consequences, and therefore the fact that superior numbers carry the day matters less.
My main experience with numbers-matter PVP was in LOTRO's Ettenmoors. Players can roll up a "monster character", basically controlling a level 50 mob. It's actually very well done - playing a stealthy Warg, shadowing the enemy raid and its healers, you really get the flavor of THINKING like a Warg, lurking until the moment is right and preferably striking in a pack. Problem is, the "creep" side needs to kill players to get rank to unlock their skills, and the game is balanced such that the creeps need a slight numeric advantage (or turtling in their NPC keeps) to stand a chance. If there are no organized groups of freeps roaming around, hordes of bored creeps wander the countryside, killing any brave or foolish adventurers. If organized groups do show up - which happens on a semi-regular basis since people are more likely to care about rank on their persistent player character than a relatively disposable creep - the freeps can steamroll their way across the zone.
There are several ways to balance this sort of situation. Blizzard's approach to battlegrounds was to instance everything and cap entry so that the sides are even (at least before AFK'ers, etc), but that inherently takes away the persistent part of the equation. Turbine's was to make several attempts to design and introduce super units - Trolls for the creeps and Rangers for the freeps - and make more of these super units available to whichever side was losing at the moment. They've had to pull the feature several times due to bugs that let people deploy more Rangers/Trolls than intended, and I didn't have time to see how the system has evolved in the last six months during the weekend's re-trial.
As to Warhammer? Well, it's a bit too early to say for sure - there are few details available, and those could all change since it is still a beta. But, with the stakes higher than you see in either WoW or LOTRO, the balance is far far more critical to Warhammer's success. Someone in the comments section at Tobold's suggested that they are considering carrots to encourage people to join the losing side (if I remember, DAOC had this feature too), but also sticks to try and slow down the winners - e.g. faction specific queues (not much unlike Blizzard's battleground queue only for the entire server).
I cannot imagine a commercially successful game telling players that they cannot play their characters because their side is too popular at the moment and having that go over well. Players want to play with their friends, not on some other faction or server. But, at the same time, the losing side is going to go from losing to nonexistent if players log in to find their cities destroyed and themselves zerged under every time they enter contested territory.
I wish them all the best, because I'd rather see more competition for Blizzard to force them to keep on their toes, but this issue is going to literally make or break their game.