Ok so I haven't racked up 100's of games yet or anything, but a couple of things have stuck out in the games that i have played.
1. Morale Phase - this honestly barely seems to matter, I have lost units to this phase, but only when they take horrendous casualties anyway... it seems like a very much wasted phase, and that includes looking from my opponents view. I can see how to use it to my effect (killing 20 orks in a unit of 30 means the whole unit is gonna die, but it is pretty limited)
2. Characters just aren't characterful, they seem to hide away giving out bubbles of buffs and not doing much else. The only time you see the Characters really hitting their stride is when its only characters on the table.
3. Spam is still strong, likely this will require some serious effort from TO's to stop it, or GW increasing points or changing how detachments work.
4. Knights (and Lords of War) are a bitch to kill, but probably aren't as scary as they seem to be at first glance.
5. Stormravens need to get hit with a points increase... 5 of them on the table top is terrifying and even only three is scary! I still see 4 Stormraven lists beating face, because it doesn't matter if you also have Guilliman on the table as he likely can't be killed till all the Stormravens are dead!
6. Characters Part II - Armies of Characters, great in Power Play / Open Play for Narrative type stuff, terrible in competitive play as the Character rules cripple units that want to shoot at different characters.
7. Net Lists - so this isnt new, its been around for a while, but I really wish people would spend the time to come up with their own lists, lists that are interesting but competitive, I think the only solution to this is to penalise people using the same list (i.e all the same units in your army) but that really becomes a pain.
8. Multiple copies of a detachement in the games, this really needs to stop as it really relates to the spam issue... how many HQ's if you take 2 or 3 Supreme commands, and three Lords of War to boot!
Rant over, guess we will see how it develops as time goes by!