The Thousand Sons video contains some beautiful artwork. I hope that at least some of that will find its way into the codex, as that would make it worth buying even if I never get around to playing a game.
The disappointment for me is this whole idea of Tzaangors being in a Thousand Sons codex, along with Mutalith Vortex thing. The whole idea of Thousand Sons is that they were turned to dust to avoid mutation. It is, therefore, blasting a massive hole through the lore to introduce further beast type units and some creature which is all about mutation. I liked the fact that the Thousand Sons didn't have units like this, and by including these beast units, it's effectively undermining the whole character of the army for me.
As for the idea of being able to field an army of Tzeentch Beastmen, that would be okay if it were a separate army. It should not be possible in a Thousand Sons codex.