Recently on BOLS, there was a discussion of how the new Chaos Codex is not competitive. You can read it here: http://www.belloflostsouls.net/2011/02/remembering-chaos-glory-days.html
If you don't want to click, I can sum up the arguments pretty quickly.
1. The new codex isn't as obscenely overpowered as the older one.
2. The flavor of the legions has been left behind in favor of renegades.
3. Several armies (mostly Emperor's Children) have invalid models (Tanks with sonic weapons)
4. Demons are generic and that's bad.
Here are my thoughts on the above points.
RE 1: Tough. Playing against armies with Daemon Princes that have 5 wounds, a 4+ invul, feel no pain, monstrous creature and ignores invuls was no fun at all. The legion armies got bonuses for taking daemon packs in certain numbers, which had no downsides. Defilers hid behind forests and bombarded the enemy. Predators with the Nurgle +1 armor value upgrade sat with impunity in the middle of a field, heedless of danger. The new codex has lots of useful options without being ridiculous.
RE 2: True, but you can work around this. I agree with Lexington that renegade Marine chapters would not automatically trade all their assault cannons for autocannons, and trash all their landspeeders and razorbacks. However, you can still play as the World Eaters. Just take a lot of berzerkers! I've seen this list work well with teleporting terminators and Vindicators for fire support. While the LAYOUT of the book and fluff has left the main legions in the dust, their spirit can be found in the list, with a little creativity.
RE 3: I totally agree. This pissed me off. Several people modded Slaaneshi tanks, Legion Terminators, and Iron Warriors players even bought a goddamn Basilisk. Now, while I think the whole Basilisk thing was a MISTAKE and am VERY GLAD Chaos is not allowed to take them anymore ever, I am angry about the other stuff. Sure Legion Terminators can now just be used as regular terminators with the appropriate Icon, but that's weak. The Legion Terminators were never overpowered, indeed they were a bit overpriced! Plus the Slaaneshi tanks had to typically pay points for a gun with as many downsides as upsides compared to the gun replaced. I'd like to see these make a comeback. Except the Nurgle +1 armor value upgrade, that was insane.
RE 4: Meh. While I agree that the old demons were flavorful and cool, I understand that GW wanted to make a Demons-only list. Should they have done this? Perhaps not. But they did it. I'd be all for a way to use the God-specific daemons in a CSM list, but haven't figured out a way to make it fair. Just infantry only. Using Steeds of Slaanesh in a CSM army was unbelievably broken.
In conclusion, the current Chaos codex has useful and powerful units without giving certain armies a free bonus for doing what they would have done already. The list is balanced and if you can't compete with it, you need to be more imaginative.
Now, the FLUFF and LAYOUT of the book is intolerable. They gloss over the main legions and concentrate on crappy Renegades. While renegades may by fun, they are the Sonny to the Legion's Cher. The vast majority of Chaos Space Marines are from the Legions, and if you want to make a renegade chapter go right ahead. However the story should focus on these stubborn hateful traitors, not the upstarts that follow their example. Plus the rules for the various marks are incredibly hard to find, as they are not with the rest of the wargear! Whoever laid out this book should be forced to watch Judge Judy, Ludovicio technique style, while tripping on acid and randomly being electrocuted in the face.