Thoughts from a game on Saturday, tons of discussion and reading the rulebook (and if anyone wants to get more games in, let me know after the 10th when i'm back in town!)
Combat is far more decisive than in years past. You're also in it a lot quicker. Dwarves can choose to fight from turn 3 on against a gunline, and previously I could have guaranteed they'd never touch me.
TLOS is huge. No penalties for most terrain is also huge. I do expect tournaments to either 1) Not have the appropriate terrain for this edition (it effectively doubled at first glance), and/or 2) have static terrain (instead of rolling for what the river is each game).
Bloodthirsters fight sooner. I have no compulsions about charging in the front now. I gain +1 CR for charging, you lose the +1 from outnumbering, you realistically won't hurt me (T6, 3+/5++, WS10), and Thunderstomp is a free +2-3 CR.
The system changes are good. New players can pick it up a lot sooner; they lowered the learning curve dramatically. Magic is still a factor; you'll realistically get 1-2 spells off a turn if you want to, and can build your army to force through 4 if need be.
My demon army will change a little. I am awaiting the FAQs on the 10th, but I suspect Demons will be okay after all. I go from 3 units of 10 Horrors to 1 unit of 30 with full command, but that's the biggest change.
2500 pt Demon army for fun, btw:
Keeper of Secrets, Siren Song
2 Heralds of Slaanesh w/ Siren Song
Herald of Slaanesh, BSB, Steed, Siren Song
2 Units of 23 demonettes with full command, Banner of can't flee/shoot
11 Seekers with full command, Banner of can't flee/shoot
9 Fiends
6 Furies
Turn 1, move up the demonettes, seekers, and keeper right in the front, move furies to threaten a war machine and fiends to flank something. Opponent's turn, 4 units in his army have to flee or charge 4 units of yours. Odds are they are within range to have to charge you (12"+move), and if they roll low you can charge them. Your turn, fiends hopefully roll up 2 units that demonettes/ keeper are holding in place. Real problem is heavy cav. Good chance of you pulling his casters into bad positions though.