1) Killing zombies using a bow and arrows is amazingly fun and cathartic. When people actually get to play it they will apologize to me for saying my Everdeen clue was for a boring new feature...
2) The new smarter zombie pathing and the number of outdoor walkers. I'm not going to say that it is back to A10 levels but there are a lot more zombies wandering around outside than in A16. And not one of them will start walking in circles. They want your brains.
3) The new systems (skills, survival, party, and quest). I know that's cheating but really I can't limit myself to three. All these new systems really have enhanced the game in my opinion and will be popular features.
I haven't been able to play a sustainable game yet to get to good horde nights but I have simulated it by spawning in 25 or 50 feral zombies who could detect me and run for my brains. It is horrific and scary. Standing around WILL result in your death. If they don't bring down your base and collapse you from where you are hiding they will gain access to where you are as blocks break and new pathways open up to you. They are relentless and you will have to actively defend and definitely have fallback positions and even an escape plan in place for when your base gets compromised and overrun.