Thunderspire Labyrinth is keen on training you up in a new type of tactical complication. It's a mechanic that players of online games know as "adds".
In 4th Edition, combats are usually divided up into encounters. One encounter is an appropriate challenge for a balanced party. Adds occur when monsters from one encounter flee to an adjacent one, and then return with allies. These new monsters are additional to the encounter's original roster - hence, "adds".
Adds are a bad thing for players. In a best case scenario, they'll make your encounter significantly harder. In a worst case scenario, they'll get you killed.
The main bad guys in Thunderspire Labyrinth are hobgoblins, duergar and gnolls. Each of these factions is both cunning and tactically competent. They will not hesitate to go for help when things go sour. If players want to make progress without being constantly on the verge of death, they'll have to take careful steps to contain fleeing foes and prevent adds.
It's an issue in the Chamber of Eyes. Areas are heavily interconnected and segregating encounters is tricky. Luckily it's a small dungeon, and the final encounter in Torog's Shrine is specifically built to rely on unavoidable adds.
It's also an issue in the Horned Hold, though. The Horned Hold features 40 monsters divided into eight quite tough encounters. The enemies are fighting on their home ground with the assistance of well-designed defensive emplacements and terrain that heavily disadvantages invaders. Players will need to brush up their skills in the Chamber of Eyes, or have some severe problems in the Hold.
Adds present a problem for the DM. The added realism involved in monsters who run for help comes at the cost of potentially exterminating parties who get unlucky or make minor tactical errors. Missing a runner is an easy thing to do, with disproportionately devastating consequences.
As written, Thunderspire's solution is generally to state that monsters will only run for help once they are the last man standing. This has the advantage of being predictable. It's also an easy situation to prepare for, as generally the last surviving monster has already lost hit points and is in base-to-base contact with at least one player.
It's interesting to note the tactical changes required to deal with runners. Stopping escaping monsters places a higher premium on ranged attacks, and on mobility. Characters with shift or teleport powers are ideal to track down fleeing enemies; low-mobility melee characters (such as melee clerics) will feel frustratingly impotent by comparison. The Avenger, introduced in the Player's Handbook 2, seems purpose-built for this very task.
It's not a topic I've seen discussed a lot on the official forums; I'd be interested to hear the experiences of others with runners and adds in 4th Edition. Leave your thoughts in the comments.