Before I go over the good/bad/ugly, I would like to point out a few major changes.
1. Marks are bought on a per model basis. This makes your squds of 5 knights less expensive and your squads of 40 marauders more expensive.
2. The will of chaos is gone. It is totally gone, and there is nothing to replace it - so if anything a BSB is even more on a necessity now.
3. Weapons and other options are generally more expensive - which can make up for the cost of what looks like a cheap unit.
So, units that will star in the upcoming book:
1. Chaos Warriors
These guys got a point cheaper. But like I said above in point number 3, they probably didn't get cheaper. Their weapon got more expensive and so did the marks if you take more then 10-15. But, because the other core got more expensive or is just not combat capable - and because these guys are just so tough - expect to be seeing warriors. Probably some unmarked.
2. Chaos Trolls
I have a soft spot for trolls. In the old book I liked them. In the new book they lost mutant regeneration (throgg still has it though) but they are a whopping 10 points cheaper, with the option to get 4 attacks with a cheap extra hand weapon or become core if throgg is taken. Vomit is no longer magical, but is still good against heavy cavalry and the like.
With the splash the new monster models are making, we might be ignoring the chimera, and boy is that a mistake. He is essentially a slightly more expensive hydra that flies. He gets more attacks then him at a higher strength as well. This guy will be great for taking out warmahcines and the like, but will be eating some units too. He does have a low LD though, so keep him near your general.
4. Vortex Beast
So while the model might not be worth $85, it is worth something. This guy is gold. The bound spell goes from good to gooder to goodest and he is no slouch in combat either with 2d6+2 attacks. Pretty durable as well. He is a threat that has to be dealt with - because if he gets off a transforming tide or a spawnchange you can destroy a battleline.
Still liking these guys. So while they changed significantly (losing 1 point of toughness, gaining 1 wound, getting more expensive) they ultimately got better. Murderous charge and cheaper lances gave them 6 s6 attacks each on the charge. If you go with ensorcelled weapons you will have your power balanced over more rounds, for a marginally higher price.
The models are expensive, I think somewhat ugly, and the rules are bad. They are a core choice, but warriors are cheaper, more reliable and stronger. So I don't think these guys will be taken.
They probably win most downgraded unit in the book. Their base cost was increased by 150%, their weapon options were made more expensive and their marks are on a per model basis run you a lotta points for a horde. A unit of 40 with flails and the MoK used to be 230 points. Now it costs 400. Still just as effective, just much more expensive.
Still random, still bad. I see no reason to ever take the generic spawn. I don't even think I need to go into more detail then that.
There are some other units which I currently think may be subpar, but I won't write them off until I play with them and review them in more depth.