Above are the stats for Wintergrasp battles on my server, Hyjal US. Each side must by definition possess the same number of captures plus or minus one, since the zone cannot fall into NPC control (i.e. you don't get to take it again until the enemy has first taken it from you). So, the only meaningful number is the number of times defenders are able to fight off captures. In our case, the numbers show a massive advantage to the Horde, which has held the keep in over seven times as many defensive battles.
In part iterations of Wintergrasp, the Alliance on my server often showed up in superior numbers, granting the Horde massive tenacity buffs. Today, it is not uncommon for a prime time battle to turn into a rout, in which my normally squishy mage is running around with 20 stacks of tenacity and 90K hit points, demolishing enemies in 2-3 hits. While this is strangely addictive (I've gotten 50-100 honor per kill in some of those situations, thanks to the tenacity honor buff), it is very unlikely result in capturing the keep. What happened to shift the balance?
The latest rules
Since the last time I addressed Wintergrasp, a year ago, Blizzard has completely redone the zone yet again. The current model requires players to either travel to the zone or two a battlemaster in major cities to queue up for the battle. This can be done no more than 15 minutes prior to the outbreak of hostilities, and was intended to reduce overcrowding. There is apparently an upper limit on how many people will be brought into the battle, and, in Blizzard's defense, I haven't seen much lag in the zone.
On the opposite side of the coin, though, the balancing of the factions is left purely to the tenacity buff. We can have a battle in which there are seven Alliance in a /who for the zone and a decent sized raid group of Horde. Unfortunately, Tenacity does not do enough to enhance the performance of vehicles - a 100K HP siege tank is actually pretty squishy compared to a 15-million HP raid boss. Without vehicles, the attackers cannot win the battle. You can stick around to farm honor points and watch insanely large crit numbers, but that will only stay entertaining for so long.
It's certainly possible that the population balance of the server has changed over the last year in ways that make the Horde suddenly outnumber the Alliance, where the situation was once reversed. The bigger issue, though, is the one that always hits games with non-instanced (or, in this case, non-size-balanced) PVP; the outnumbered side starts losing and becomes less fun to play, and even fewer people show up, starting a vicious cycle.
Consequences of the NEXT revamp
Blizzard's plan for the expansion (to be tested in Wintergrasp and finalized in the expansion's new version of Wintergrasp) is to limit both sides to relatively equal number of players (with a minimum cap to ensure that one side cannot deny the other victory by refusing to show up and leaving the cap at some number that's too low to complete the objectives).
Like the last half a dozen iterations of Wintergrasp, this one has some problems. Off the top of my head, the more popular side may quickly realize that they are less likely to get in off of the queue due to their numbers and level alts on the opposing faction to enter the battle just to raise the population cap for their real comrades. With Blizzard's new account-wide chat feature, they can even be relaying intel about enemy movements through in-game whispers. There's no stopping players from using third party chat to accomplish this, but it hasn't been conveniently and officially in game before now.
The bigger issue, though, will remain how to keep this kind of PVP - in which one side, and often the same side, will lose more often than not - interesting enough for the losers to choose to continue. This is where Warhammer fans have always claimed that the game went wrong by not emulating DAOC's three-faction model; you might be outnumbered, but there's always the chance of the two smaller teams joining forces against the big guys.
In the absence of changing the system to make sure that victory is always somehow in reach, Blizzard has attempted to use incentives to keep the losing side happy. In Wintergrasp's case losers can snag maybe 1-2K honor and a token (good in quantities of 25 or 40 for a second-tier PVP item, and only redeemable if your side owns the keep), with additional points for any of the weekly quests you are able to complete. Apparently, in an era where all the other forms of gear have also seen massive inflation, those rewards aren't cutting it.