What will be very interesting to me is the new Psychic tree.
I disagree with some of what GW has been doing to psychics for other factions. I truly hate removal of core functions like "Obsec". And I can see that GW has been toiling away in that area for easy 'power gains'. Nothing makes something better, and quicker than having a power that just lets you pretty much point at something and say, "That unit loses X". X could be leadership, or could be ObSec, or could be movement, or could be the ability to fight in chronological order, etc, etc.
I fear that the way power creep has gone in 9th (and I think it is WAY higher in 9th that it was in 8th) that there maybe a fist full of powers that are just meddling in those 'core' areas just to give Thousand Sons a valid playground in 9th.
I say "I don't like it" for two reasons: 1 I mentioned this, but I hate all this "You fight last, no you flight last, no you can't fight until I'm done". Removal of Obsec is something I really hate. It's why we (largely) take troops. Leave it alone. Even movement can be just so penal to zap from certain armies.
The second reason is because these abilities are so game changing, they will have hard counters. Look to the plethora of new 9th ed abilities to outright cancel a psychic power to see what I mean. Dark Angels for example... those watchers in multiple squads are just too good.
Of course I digress, this is just pure speculation on my part. I just have not been a fan of the psychic phase in this edition. I don't like how it works, I don't like the order of it, the picking of power during the "mustering phase". It really makes it far less dynamic. Perhaps that would be one of the best things you could flat out do for Thousand Sons; let them pick powers just before battle round 1 begins. (I know that's a pipe dream).
Anyway, I struggled with Thousand Sons just because dice games can be so maddening. It was part of the attraction to Thousand Sons... the warp is fickle, right? Some games you can't miss, the next game you could blow your whole tournament on the fact you just couldn't get a power off for 2 turns or something (I think this has happened to me more than I care to admit.)
But for me, this whole book boils down to the Psychic phase/ Psychic tree more than anything else.