In a battleground group, it's basically healers and everybody else. There are many roles to fill, depending on the battleground, and it'll be up to you to assign the players. Sometimes, some classes are better for the role than others, but battlegrounds will tend to be forgiving of team composition as opposed to the more restrictive arenas. Healers are the cornerstone of any good battleground group, but how you assemble the rest of the team is pretty important, too.
Flag carriers Druids have traditionally been the most ideal flag carriers in Warsong Gulch (and soon Twin Peaks) because of their superior mobility and their ability to take a beating when in Dire Bear Form, plus they can also heal in a pinch. With Stampeding Roar as part of their arsenal in Cataclysm, druids seem a lock to become staples in capture the flag groups. Teams can also opt to assign the role to tanks designed to withstand focus fire. Unlike arenas, flag carriers don't necessarily need to hit back so they can focus on survivability and mitigation talents and gear -- think of them as raid tanks who stack on resilience and stamina. Warriors make excellent alternatives to druids because of their mobility (clever use of Intervene) and their ability to hamper their opponents' mobility (Hamstring) or crowd control them (Intimidating Shout). Second to healers, flag carriers in capture-the-flag maps are the next important piece of the puzzle.
Snipers Your ranged DPS are the snipers. Hunters, mages, warlocks ... your team will need a decent complement of players who can attack from long range. The ability to engage the enemy from far away is good to have in an environment that allows significant freedom of movement. While hunters normally suffer in the confines of arenas, they thrive in the battlegrounds. Although not quite a necessity in the same way as flag carriers or healers, snipers or ranged DPS are great to have in, say, Alterac Valley. This isn't so much of a specialized role as it is an awareness of what ranged DPS can do in particular maps -- for example, ranged DPS can do significant damage with little fear of retaliation attacking from the ledges in Strand of the Ancients. Ranged DPS should also know to refrain from picking up a vehicle lest they waste their ability to attack from inside one.
Drivers Speaking of vehicles, players must know how to pilot the various vehicles in Strand of the Ancients and Isle of Conquest. A better understanding of vehicle movement and combat, as well as fair knowledge of the abilities should be the bare minimum for all team members. Glaive Throwers, for example, are excellent siege vehicles and knowing how and when to use Glaive Throw or Blade Salvo sets good teams apart from mediocre ones. At the very least, lesser geared teammates can use vehicles as a buffer. Use of vehicles also includes knowledge (and skill) of how to use a stationary turret in, say, Strand of the Ancients.
Defenders These are the guys who stay at the flag. More than anything, these are players who have very little ego, willing to sacrifice their personal numbers -- killing blows or whatnot -- for the unglamorous role of defending a node. This is important. Oftentimes it takes just one or two players to stay at a node in Arathi Basin or Eye of the Storm, and sometimes it happens that nothing happens. It's far from the action, but node or base defense is just as critical to winning the game. In Alterac Valley, these are the guys who stay at the bunkers and towers and wait until the thing burns down. Players who can spot enemies from a distance, such as hunters and druids who can track humanoids on the mini-map, make excellent defenders.
Commandos For the most part, teams work best when moving as one unit. Sometimes, however, there are those players who perform exceptionally well on their own, possessing a high enough PvP IQ that they can be left to their own devices. Rogues and druids, the classes with stealth, are capable of moving around the battlefield without getting noticed. In some cases, these commandos can retake a flag in Warsong Gulch after laying in wait in the flag room while stealthed, preventing what would otherwise have been the enemy scoring a point. These players make great scouts and can even "ninja" undefended nodes in Arathi Basin.
Assembling your ideal battleground group should be a lot of fun, and will be much less precise compared to forming arena teams. More important is understanding the roles that one should play -- and roles change depending on the battleground, so a sturdy flag-carrying tank might be the center of attention in Warsong Gulch or Twin Peaks, but they'll be remarkably less useful in other maps. The more we learn about the abilities in Cataclysm, the clearer our picture will be as to what classes can perform best in what role. By the time the game is released, we're sure to learn about cool abilities that should determine a class' desirability in a team and what roles they can perform better. For now, all we can do is dream about our perfect team.