For the record, the allies rule states that Battle Brothers (the highest level of allies) count as friendly units for all purposes. So rules which state that it applies to friendly units (e.g. Fateweaver's special rule) get the benefit. Rules which say something about a specific type of unit (e.g. Tau Markerlights) dont carry over. At least until an FAQ says otherwise.
Noticeable changes:
*You can leave some models in a unit stationary, whilst the rest move. This means you can move some models but keep your heavy weapons ready to fire. (Must remain in coherency though).
*Mixed armour save units are going to be an absolute pain to work out now, even more so if you have mixed AP weapons in the firing unit. Also, the firer gets some control over how to allocate wounds now (this isn't as good as it sounds, but better than before).
*Models out of LoS cannot be wounded by attacks on their unit.
*Look Out Sir is a 4+ save for characters (sergeants etc) and 2+ for ICs. Note that Mephiston only gets a 4+ as he isn't an IC
*Cover saves are done on a model by model basis. You need to have 25% of the models body covered from the point of view of any of the firing models. Cover is generally a 5+ now. You can choose to ignore models in cover and hit the ones in the open using the "Focus Fire" rule
*Assault is now 2d6 rather than a straight 6". Charging through cover is 3d6 take the lowest 2.
*During each initiative pass in the assault phase, unengaged models from BOTH units pile in 3" before striking blows.
*You check to see who can fight at each initiative pass. This means that if the powerfist armed sergeant is further than 2" from any enemy models by the time you get to Initiative 1, he doesnt get to strike.
*You can choose to auto fail an Ld test if you lose combat against a model you cannot hurt. (No more locking up units with Wraithlords or Dreadnoughts)
*If you assault multiple units, you dont get +1A for charging.
*Falling back units are BS1. They auto pass all pinning tests.
*You can regroup if 25% of you unit is still alive. If less than 25% remains, you can regroup on a roll of double 1. When you regroup you still count as BS1 and cant move/assault.
*You can regroup even if there are enemy models within 6".
*New USRs covering Bulky models (count as 2 models for transport), Very Bulky (count as 3 models) and extremly bulky (Count as 5!!)
*Fearless no longer causes extra wounds if you lose close combat, but you cannot go to ground and you cant choose to auto fail against models you cant hurt. (see above).
*Feel No Pain is 5+
*Fleet lets you reroll 1d6 for a run move or charge, but doesn't let you run AND charge.
*No Initiative bonuses from Furious Charge any more. You still get the Strength bonus.