Tol Barad is Cataclysm's equivalent of Wintergrasp. It is an island off the coast of Gilneas where Horde and Alliance will vie for dominion of Baradin Hold and the raid instance beneath it.
Tol Barad is divided into two sections: the main island housing the Tol Barad Prison complex, and the peninsula to the north. These two areas are connected by a bridge on the north end of the main island. Tol Barad peninsula will serve as a daily questing area similar to the Isle of Quel'Danas. Whichever faction controls the island will have access to even better daily quests and rewards.
The island of Tol Barad is a magic prison that houses dangerous mages, demons, warlocks, and undead criminals. The island was long kept secret, but now is a battlefield for Horde and Alliance forces trying to gain control of its strategic location.
The layout of the island is fairly simple as are the objectives of the battle. There are three major prison facilities: Ironclad Garrison, Warden's Vigil, and the Slagworks. The attacking forces must establish control of all three of these facilities simultaneously in order to take control of Tol Barad. Gaining control is done in a king of the hill style identical to Eye of the Storm where players stay in the area in order to tilt a bar in the direction of their faction.
Obviously, given the king of the hill style, it wouldn't make sense for there to be unlimited numbers of players allowed into Tol Barad at once. Otherwise the outnumbering faction could simply sit in an area so that they would never relinquish control. Blizzard plans to have a queuing system for Tol Barad which will keep the number of players from each faction in the area very close together. This is going to be a disaster for realms where the population is highly skewed in favor of one faction, but it's the only way to make it a fair fight.
Tol Barad's design is simple and elegant.
There are three towers in the North-East, North-West, and South areas of the map. These towers are similar to the ones in Wintergrasp except they serve the opposite function. Whereas in Wintergrasp the defending faction destroyed the towers to shorten the length of the conflict, in Tol Barad the attackers kill the towers in order to put additional time on the clock.
Tol Barad tactics revolve around the defense managing their forces in order to control at least one of the three bases at all times and keep the towers fairly well defended. The offense must find a way to disrupt the defense long enough to control all three bases at once.
Conceptually, Tol Barad is much more complicated and much better than Wintergrasp. It's also much more difficult for the offense to win, the defense has a definite advantage. When the battle begins there are 30 minutes on the clock. The timer counts down and at 0 the defense wins. Each tower that is destroyed by the offense will put an additional 3 minutes on the clock. Simple enough.
The real difficulty is in establishing control of all three bases at once. As you can imagine, it's roughly equivalent to the difficulty of 5 capping Arathi Basin or 4 capping in Eye of the Storm except that in this case the defense knows that all they have to do in order to win is hold onto just one of those bases. It's definitely an uphill climb for the offense.
The difficulty of accomplishing this is compounded by the fact that the Tol Barad map is big. It's not as big as Wintergrasp, but it will still take you a solid 2-3 minutes to ride from one side to the other making it absolutely vital that your team be coordinated and working together. If you're on offense and your opponents are turtling in one base then you will have to rally the troops, take time to meet up, then attack as a group or you are going to be unable to dislodge them. Furthermore, if you attempt something like this you will need to have players stationed near the other two bases to alert you if your opponents try to shift their forces to another base instead. Gaining 100% control of a node from 0% takes very little time when there are 10+ players in the area, perhaps 20 seconds, so it's vital to always have forces ready to respond and defend when your areas are under attack.
Want to kill a seige engine before it takes down a tower? Good luck.
The three towers are a useful way to buy time and don't require an awful lot of effort to take down for the offense. Scattered around the map there are abandoned siege vehicles that move extremely quickly, have obscene amounts of hit points (in the vicinity of 1.3 million), and are heavily armored. In the current state of the game it's almost an exercise in futility to try to defend the towers because the siege vehicles are so durable. You don't need to invest too many players to take the towers down, and it's important to remember that even if you've bought a little bit of time that won't save you if you're unable to break the defense.
Tol Barad's objectives are still being balanced, and I suspect that the siege vehicles will be nerfed a bit so that they're killable in a reasonable time frame. But even so, the defenders are at a distinct advantage in this map simply because they only need to control one base to win whereas the offense must control three.