FMC have taken a bit of a nurf with both the not being able to charge after changing flight mode that turn, and smash becoming single attack rather than half attacks.
Many people will claim that tzeentch armies are stupidly powerful because of dominating the psy phase what is kinda true, but also weirdly wrong depending how you play them.
Depending who you are going against really depends what you want in your army though.
Stuff like bloodletters and bloodcrushers are great vs MEQs, but Daemonettes are so much better against orks and guard. Hounds of Khorne are normally a must, and Beasts of Nurgle are insanely tough to kill.
There are a few units that normally it is worth not getting like Furies.
With one of the new major changes having loads of heralds seems like a good plan as you are no longer limited to having only 1 of your HQ choices per army allowing 4 heralds, it is now 1 HQ per FOC. And with the limitations on how many FOC changing from only allowing 1 per 2000pts or part of, to now just being 1 FOC per minimum requirements being filled (1hq, 2 troops). This allows a load more flexability for someone who doesn't want to take a MC or FMC as leader.