Well, Battlegrounds has been out for almost two weeks. And I'm already bored with them.
There are currently two Battlegrounds: Warsong Gulch, and Alterac Valley. Warsong Gulch is a game of capture the flag, while Alterac Valley is a battle for control of the valley.
Capture the flag suffers some immersion problems in the Warcraft setting, but works for the most part gameplay-wise. Alterac Valley has the opposite problem - the setting makes sense in the World of Warcraft, but the gameplay gets frustrating fast.
Both battlegrounds suffer from the supposedly non-existant population imbalance. The overpopulated side gets massive queue times, and the underpopulated side frequently finds themselves outnumbered. It's not unheard of for a battle to start 40-20, and then rapidly end when the side with 20 people decides it's not worth sticking around for.
Then there are various other problems. Warsong Gulch consists of quick games, while Alterac Valley is an "epic battle." With way the current system works, you get more honor for winning a game of Capture the Flag than for winning an "epic battle." That's just not right.
Plus Alterac Valley is virtually unwinnable. Both sides get a choke point where it is far easier to beat off defenders than it is to break through. Of course it should be hard to take an enemy base, but it shouldn't be nearly impossible.
The real problem is the respawn rate, which is simply far too fast for both players and NPC guards. Players respawn every 30 seconds at practically full HP/MP. This means that even if your side defeats the other side, you'll be facing them again in 30 seconds. (This isn't completely true - technically only 10 ressurrect at a time. However, it's basically impossible to kill the other side such that they have more that 10 waiting to ressurrect at the 30-second interval.) The NPC guards respawn every 5 minutes or so. This makes the already long time to fight back the continually respawning opposition even more annoying as guards you've already killed come back to life and start attacking again.
Plus, there are guards that patrol the middle of the map that respawn even after the opposing side has cleared the base they inhabit. Each side has two "towers" that can be taken that should stop the spawn of guards in the middle. However, they do not do so, so traveling back and forth between the front line and your base becomes a giant hastle as swarms of NPC guards zerg you.
There's also the problem with "ninja-looting" corpses. When someone dies in the Battleground, the opposing faction can "loot" their corpse and get items that are useful for gaining "faction" with your side. The problem comes because melee units are up close and personal with the enemies, while ranged units are standing further back. Since anyone can loot, this usually means that ranged units get nothing while melee units get massive faction gains.
Blizzard has announced changes to Battlegrounds which may improve the experience. So just because the current incarnation is essentially a failure does not mean that the game will always lack fun.
They're addressing the ninja looting problem and the NPC guard spawn, but they're not addressing the lack of death penalities. The game already has an increasing timer for multiple quick deaths. If you die, you must wait 1 minute before you can res - unless you're in Battlegrounds, in which case you'll get ressed at 30 second intervals, regardless.
It's too soon to say Battlegrounds will remain a failure, but despite the initial surge of activity, many people have found Battlegrounds to be lacking and have simply stopped playing them.