If you’re still making noises about Allies: stop. I’m tired of hearing people complain about/try to break the rules.
Much more significantly, it’s not the biggest rule change in the new book.
“Bigger Games”, page 110:
This being the case, if you’re playing a game of 2,000 points or more, you can take an additional primary detachment. This gives you access to up to six choices each from elites, fast attack and heavy support, up to four HQ choices, a massive potential of up to twelve troops choices, and additional allied detachment and an additional fortification.
This is huge. It means 2,000 point games are explicitly not in the range of default game sizes. It means crazy shit like 8 HQ choices in a 2,000 Space Wolf army. It means I could drop over 50% of my points on HQs in my Daemon army.