I'll just dive right into it with some random thoughts.
At first glance it appears blob units have gotten a boost with the chaos cult units. A Dark Apostle can make them rock hard to displace off an objective and Typhus can make them fearless. But while Chaos now has the new Blob unit of 6th edition, I think the codex has also given us a tool to defeat all blob units everywhere. 10 Khorne Berzerkers will deliver an average of 22 wounds to blob units of cultists or guardsman. Throw on chain axes and all of them get no saves. Plague marines can dish out 30 attacks on the charge, but more importantly can only be hurt on 6's from the blobs. The power axes hurt the plague marines on 5's, but they still get their FnP save. Even the basic chaos marine unit can be built do dish out 40 attacks and average 18 wounds for only 170pts.
The point is there are many choices in this codex for dealing effectively in combat with blob units. Will people take them? That's another story.
Next are the Obliterators. They can get toughness 5 and become immune to instant death from str 8-9 weapons. This is how they were at the beginning of 4th edition and that made them very powerful. Well now they are back to that power level. One new thing about Obliterators is they are no longer in terminator armor which means they can ride in rhinos. It is also possible to shove 3 of them in a Bastion and make them immune to return fire while all three can shoot out with impunity. Three of them can also fit on top of the bastion so attach an IC to them who mans the Quad Gun and the unit has a 2+ armor and 3+ cover save. Not to shabby, but they are safer inside the bastion.
I'm not sure what to think about the new daemon engines. Their BS/WS of 3 is a turn off so is their armor of 12. I suppose if people insist of take psybacks and psycannons, then the Daemon engines will be a struggle to take down. Maybe all they will do is keep people honest and prevent every meltagun turning into a plasmagun.
I like Huron Blackheart for two reasons. One he gives d3 infantry units the ability to infiltrate and thus outflank and two he is not that expensive. I like the idea of outflanking either Obliterators or plasmagun toting havocs or a large unit of cult troops or a big unit of terminators. Now we are talking.
The babies crying and I have to go.