Daelo - the Lead Encounter Designer - posted on the Raids & Dungeons forums last night about some known issues with the encounters in the Battle for Mount Hyjal(also known as Hyjal Past, etc.). The original post can be found by clicking here.
I wanted to give the community a heads up on some issues that are occurring within this zone, and let you all know what's being done to address the issues.
Archimonde currently has a very short leash distance to prevent players from bringing him near the World Tree. The waters of the Well of Eternity could be used to circumvent a great deal of the challenge of the encounter, so as a short term measure we implemented the rather aggressive leash. The leash makes fighting him more difficult than we intend, and this leash distance will be further expanded when we're able to add a new spell to the Well of Eternity in an upcoming patch.
NPCs killing Bosses
Hyjal is a zone that has you fighting along side Jaina, Thrall and their Alliance and Horde forces against the invading armies of the Burning Legion. The NPCs can be a big help when fighting the Legion. They provide the raid with a supplementary source of damage, and occasionally even tank a Ghoul or Abomination for you. This is great, except for one case. If your raid attacks one of the bosses, then damages the boss to a very low health amount, the NPCs can continue on with the fight and finish the boss even if the raid has wiped out. Thrall is particularly good at this, as he has an immense amount of health and summons Dire Wolves to help him. This is a problem because the raid can lose tap on the boss while they are all dead. This then means the raid can no longer loot the boss. The raid at this point can also not retry the encounter, as the death of the boss has moved the zone's progress further. The raid can continue on and fight another boss, and eventually even Archimonde, but that loot is no longer available for that week.
We have a very strong fix implemented for a future patch that should prevent death of the boss if only NPCs are fighting the boss. This particular change could not be hotfixed, however. To reduce the chance of the NPCs winning after the players are dead in the short term, we have greatly reduced the hit points and damage that Thrall and Jaina deal via a hotfix that is now live. Raids are advised to be more careful about letting Thrall or Jaina take damage, since the death of either results in a loss.
Burning Legion Waves
In the same patch we add the fix for NPCs being able to kill the boss, we're also going to be reducing the number of waves before each boss from 12 to 8. Some of the largest waves have also been reduced in number of creatures. In an even later patch, we'll also be adding loot (including epic drops) and faction to the creatures. This should hopefully take some of the sting out of losing to one of the bosses by making wave clearing a less time consuming and more rewarding experience.