Things that people don't complain about in AoS:
- Rules for free
- Rules that come with the units
If they do that with 40k, then that'd be great. No need to buy every book, Woohoo!
Things that people complain about in AoS:
- Rules wording that leaves game changing details up to interpretation
- Lack of any balancing mechanic in the game
The rules wording isn't so much worse than it in in other GW games, so that's not such a big deal. One FAQ and we're done.
But if Games Workshop takes all balancing mechanics out of 40k, they should add a suicide note in the back of the rulebook.
That's not to say that they couldn't conceivably balance the game with formations- but they need to put forth an effort to balance the game.