The allies addition is new system that allows player to have a detachment made of another army in the list. Allies works like WFB 8th Edition with different 'levels' of allies - Brothers in Arms, Allies of Convenience, and Desperate Allies. The detachment requires 1 HQ and 1 Troop (compulsory) which opens up an additional 1 Troop, 1 Elite, 1 Fast Attack, and 1 Heavy Support. Allies do not benefit from each others special rules, hitch rides in each others vehicles, etc. Brothers in Arms and Desperate Allies carry their own special rules as well. The force charts also have a new entry for 0-1 Fortification which allows you to buy a terrain feature as part of your list.
I have absolutely no doubt this will open up all kinds of headaches and munchkinry for competitive play, but as some one who collects multiple armies and enjoys games almost exclusively at home, I welcome the chance to field my Space Marines and Imperial Guard together as a 'legal' force, reminiscent of the old school Rogue Trader army list. With the arrival of 6th edition, I went from having four 1500 to 3000 point armies to two 4-5000 point forces depending on how the details work out. I'm not counting my new Red Scorpions and Tyranid armies in this list, though I understand Tyranids aren't allowed to have any allies - probably difficult when you tend to eat everything that moves! Again, I don't know how that works out in competition as it seems like a distinct disadvantage for the 'Nids, but I could care less about perfect balance for games at home.
EDIT (June 27, 2012): Confirmed Allies List for my Imperials:
SPACE MARINES ('Vanilla' chapters like my Crimson Fists and Red Scorpions)
- Brothers in Arms: Black Templars, Blood Angels, Dark Angels, Imperial Guard, Space Wolves, and Tau (I don't get the last one)
- Allies of Convenience: Eldar, Grey Knights, and Sisters of Battle
- Desperate Allies: Dark Eldar, Necrons, and Orks (I guess everyone's on the same side when the Tyranids come to eat you!)
- Brothers in Arms: Black Templars, Blood Angels, Dark Angels, Sisters of Battle, Space Marines, and Space Wolves
- Allies of Convenience: Chaos Daemons, Chaos Space Marines, Eldar, Grey Knights, Orcs, and Tau
- Desperate Allies: Dark Eldar and Necrons
Now, I won't be fielding my Death Korps of Krieg with Chaos Space Marines, but I think it's very interesting that you can model a traitor Imperial Guard force with Daemons or Chaos Marines in tow. I don't think that Space Marines should have Tau as Battle Brothers either (the 'name' Space Marines chapters like Black Templars, Blood Angels, etc. only count them as Allies of Convenience). I'm not a min/maxer enough to search through and find out which famous Space Marine chapter(s) to add detachments from for my regular Space Marines, but I can definitely see building small detachments of Eldar to supplement my Imperials!
Historical gamers fear not - I have two more Grand Alliance units on the table and more on the way! I have not been turned to the dark side completely, but with 6th edition due out very soon I have payed a lot more attention to GW than usual.