Blizzard plans for the Battleground to have ten or fifteen players per side, with the map very close in size to Arathi Basin, Eye of the Storm, or Warsong Gulch. They plan to throw up the gameplay by having a side defend for one part of the battle and then attack during another. Players will need to break down walls using siege weapons, with key capture points past each set of walls allowing access to more siege vehicles.
The map design is unlike previous Battleground maps which were mostly symmetrical. The Northrend Battleground is uni-directional, oriented towards one goal that players must assault or protect -- presumably the red dot on the upper right hand corner of the illustration below. From what we can see, there seem to be three graveyards across the map, with a hotly contested central graveyard that can turn the tide of battle. Flags appear to be capture points while there appear to be three existing siege weapons.
Blizzard revealed that the map is currently undergoing playtesting, which could mean that it is the most far along in development. Since siege weapons in Lake Wintergrasp also seem to be the most developed, playtesting the Battleground at this point certainly involves them. It's an interesting map, to say the least, and I'm not sure how the game will reverse the roles of attackers and defenders. But the terrain itself is a challenge... for attackers, it's all uphill, while defenders will have an easier time looking downwards.

The verdict
This looks like Blizzard's most dynamic instanced Battleground to date, not only with the addition of destructible buildings and siege vehicles, but because of a uni-directional orientation. Presumably, players won't know which side they'll be on when they zone into the Battleground, necessitating an adjustment each time. The learning curve can be tricky, specially if players keep getting the same side (offense or defense) several times in a row. There's still too little information to judge, but the map already seems exciting from what we've seen.

Defenders can't turtle when there are capture points across the map that allow access to presumably stronger siege weapons which can break the turtling. The central graveyard grants a strategic advantage that there's likely to be lots of combat in the central area, but enough points of interest to make it hard to defend all of them. There can be no races because those on defense have nothing to race to. It's a very unique dynamic unlike anything we've seen in Battlegrounds so far... each side has different goals. This asymmetry looks like it could work. My only concern is the length of time games will take, considering the added complexity of siege warfare. Hopefully we'll see this Battleground in play, whether though Beta or a trailer, soon.

The Q&A
There weren't very many questions that got through, considering the length at which Chilton and Stockton discussed Lake Wintergrasp, the new Arena maps, and the Northrend Battleground. Coupled with French translation, only a few interesting points were raised... and none of them about Arena class balance.

One of the concerns was how Alterac Valley was giving more Honor than other Battlegrounds (as the questioner put it, other Battlegrounds were played mainly for Marks of Honor) and how Blizzard planned to address this. Chilton seemed to misinterpret the question (deliberately, perhaps?) and talks non-sequitur about the AFK reporting system. Unfortunately, it doesn't seem as though Blizzard will increase the Honor gains from other Battlegrounds to balance Alterac Valley's Honor-per-minute output. AV remains the most efficient Honor-farming ground with no changes on the horizon.

An interesting question came from a couple that ground to High Warlord together (and hit Rank 14 the same day!), asking when they would be "free to play Battlegrounds." They were asking, in essence, if they will be forced to play Arenas just to get gear. With the new personal ratings requirements in place, even Honor-bought gear from Season 4, require participation in the Arenas. Chilton's unsympathetic (though not verbatim) answer was, no, you have to play Arenas. If you just want to play Battlegrounds, settle for the neckpiece and bracers.

This seems to be a trend in Blizzard's itemization philosophy. The best gear will come from Arenas. This is going to be detrimental in the long run as players are pigeon-holed into playing a form of PvP that isn't necessarily enjoyable to them. Players who wish to only participate in the Battlegrounds must wait for the current season's gear to devalue over the course of almost a year before being able to purchase them. How this bodes for the non-instanced Lake Wintergrasp remains to be seen. Hopefully, the Wintergrasp rewards are compelling and substantial -- even upgraded over time similar to Arena seasons -- in order to encourage participation.

The verdict
At the moment, Blizzard's focus seems to be on the PvP environments, rather than the participants themselves. This isn't to say they're not looking into it, however. New talents given to some classes such as the Priest's Dispersion, for example, indicate that they see a problem and are taking steps to fix it. PvP balance has always been a concern for Blizzard, particularly now that they're so enamored with Arenas. I am optimistic that Blizzard will address class and spec balance in time. Right now, I think, the developers are having fun with things that make other things go boom.

Read about Zach's analysis on the Lake Wintergrasp and new Arena maps discussions!