As you can see on the map above, the Horde and Alliance each have two major territories, one for each captain and general. Each territory has its bottlenecks that force attackers and defenders into certain places, and three of the barriers involved can be passed going one way but not going the other. Each team's offense must progress through two stages in order to reach the opposing general and destroy all the towers. Both the Horde and Alliance ideally have some players on offense and some on defense, but as we'll see shortly, these territories are far from equal when it comes to how easily they can be defended. The Horde has a very strong frontal defense near their captain, but a somewhat weaker defense near their general -- the Alliance on the other hand, has a very strong defense around their general, but their captain's territory is effectively free for the taking.
To the left we see the Alliance captain's territory, Stonehearth. The entire area is wide open except for a narrow pass in the north, just near Icewing Bunker. There is minimal natural terrain blocking off access to any of the objectives, or forcing the attackers into a certain position. The graveyard can be approached from two major directions, and it is comparatively easy to capture. Defending it simply takes too many players, so Alliance usually doesn't bother. Stonehearth bunker, in particular, is so far south that the Horde can actually reach it first at the game's outset. If the Alliance wants to defend it, the only way is to take it back after the Horde has already been there.
Iceblood, on the other hand, is mostly closed off to entry from the north. While the Horde captain and the first tower are accessible, the Iceblood graveyard is nestled behind a convenient hill, creating a bottleneck very close to the resurrection point. Just a few Horde players defending any objective from there can seem like many, because after they die they come back to the battle so quickly. By contrast, the nearest graveyard to the north is the neutral point, Snowfall, which the Alliance must wait 4 minutes in order to actually make use of, and which is farther away from any Iceblood objectives in any case. At the start of the game, the Horde can wait for the Alliance in Iceblood tower, the graveyard bottleneck, or their captain's bunker, and then get right back into the action within a minute if they die.
Iceblood graveyard is by far the strongest place on the map for the Horde. It is perfectly positioned for defense of the Horde captain's territory, as well as for an offensive push into Stonehearth. In addition, even if the Horde does not defend it initially, Iceblood is relatively open for them to recapture from the south, even after the Alliance forces have been there. This recapture is especially easy if the Alliance offense is split up trying to kill the Horde captain and take both the two towers. Once the Horde taps Stonehearth graveyard, the entire Alliance offense is sent back all the way to Dun Baldar.
This Alliance home base is much easier for them to defend. Its strength matches or surpasses that of Iceblood. Before patch 2.3, Horde would find themselves having a hard time at the Stormpike graveyard bottleneck, and an even harder time getting past the Dun Baldar bridge into the Alliance general's base area. When killing the general was all that mattered, the Alliance defense could do very well just holding off the enemy here. But now that running out of reinforcements ends the game, the Alliance no longer has sufficient time to get enough players back on offense once their first attempt has failed. Remember that bottleneck in the northern half of Stonehearth? It has no use for the Alliance, but now the Horde can use it to lock them completely inside Dun Baldar. Not only is Stonehearth difficult to defend from the initial onslaught, but in the hands of the Horde, it prevents all but a trickle of Alliance players from having a second chance. The Horde thus secures a guaranteed victory by taking at least two towers and killing the Alliance captain, then fighting as far as they can till reinforcements run out.
The Horde general's area, on the other hand, is certainly weaker than Dun Baldar, but this hardly matters once the Alliance has been trapped north of Stonehearth. Only if the Horde is too late to defend Iceblood does the southern terrain of their territory turn against them. The Alliance can attack the Frostwolf graveyard from multiple directions if they have enough players in the area, or they can just ride past the graveyard into the Horde general's home base. There is a bottleneck leading into this base, but it lacks the direct line-of-sight advantages of the Dun Baldar bridge, and it is a bit more difficult for the Horde to take advantage of. None of this matters, however, if the Alliance doesn't have enough numbers there, or if the Horde offense has already made a strong push into the Dun Baldar area. If the Horde sets the Alliance back at Iceblood -- and they can without too much effort -- the game is already over, though it may take 20 or 30 minutes of turtling to finally run out of reinforcements. In the end, the Alliance will get very little honor, and the Horde will get a great deal.
Some people will try to tell you that the terrain doesn't matter in AV, that just like the other battlegrounds, it is all about player skill and gear. Certainly before and after the changes in patch 2.3, there has been a lot of AFKing on one side or the other, as well as many players who whine, or don't even try to win if they perceive their faction has even a slight disadvantage. But according to current reports from players, we can see more than ever before that one side has the opportunity to completely shut out the other, to take all the bonus honor for themselves and leave none for their opponents. It's less a matter of skill, and more a matter of whether the Horde decides to defend Iceblood or not. Even Blizzard agrees, and has started making minor changes to AV already.
In those battlegroups where the Horde and Alliance have a somewhat more even win/loss ratio, the Horde is choosing to race to the finish rather than shut the Alliance out at Iceblood -- effectively giving the Alliance a chance to win on purpose (or else out of ignorance of their advantage). In those battlegroups where the Alliance no longer signs up for Alterac Valley, and the Horde win 100% of the time, they are relishing in the ease with which they can set up their own northward attack and destroy the Alliance offense in just a few important battles near the beginning of each match. The Alliance decides not to play a hopeless game, and the Horde has to wait an hour or two in the queue for each guaranteed win.
Unfortunately, if the Horde want to keep playing Alterac Valley more than once every two hours, they have to start by giving the Alliance a chance to get going at Iceblood and at least get some honor for themselves. Horde who shut out the Alliance completely may feel very happy with themselves -- perhaps even justifiably so in some cases -- but they're doing themselves a disservice in the long run. Losers who gain nothing from a battle don't come back to lose over and over again, no matter how much they love the game.
In principle, the addition of reinforcements is good for Alterac Valley -- the possibility of an indefinite stalemate is bad for any battleground. But the map was designed around a "kill the general" game, not what we have today. Thus, changing the rules for winning the game has completely changed the game itself, and a previously minor map imbalance has become the most important spot in the game: Iceblood graveyard is the new pivot of Aterac Valley, and the Horde gets there first.