The spoiler is out! So, you know...spoilers ahead.
Embalm is a pretty interesting thing to me, from a flavor perspective. White zombies just don't happen unless Black is involved. From a plane where death is considered a positive though, this makes sense. Aside from that, however, it's just Flashback for creatures. Nothing wrong with that but nothing super exciting either.
Although I will say that having a way to get a creature despite countermagic is a positive. It could mean some better countermagic is coming (a positive for control players who have been complaining for a decade now), as well as an opportunity for older formats, too. We shall see.
Cycling is an old classic making it's 4th appearance. Cycling is always a good thing. It's well balanced and offers a lot of options to players at every stage of the game. Now that WotC has given us reminders that cycling triggers discard effects, I wonder if some novel or previously overlooked interactions will bubble up?
-1/-1 counters I am excited about! I've always liked this mechanic and believe that there's more room to develop or play with than one might think. Partly because of the interaction between +1 counters (a + and a - will, just like in math, cancel each other out) so there's some fun to be had with mechanics from other sets, partly because there is an opportunity to do some powerful cards with a drawback players have to be clever to work around.
I liked it in Shadowmoor/Eventide and I like it here.
Aftermath is an excellent marriage of split cards and flashback. Nothing too fancy, mind you but definitely something I hope continues in Hour of Devastation* (which is the next Magic set, just in case you hadn't heard). Solid ideas don't have to be flashy, they can just be solid.
Exert...is...meh. It just is. There isn't anything clever about working around the drawback (untap your critturs or give them vigilance) and the only time to really use the ability is when you're already winning. There isn't going to be some kind of trap set up by your opponents causing you to say "oh, if only I hadn't exerted!"
Smaller themes I noticed:
Enchantments matter again. Seems like the gods of Amonhket manifest their powers through enchantments and I am digging it.
Tapped creatures matter. Yeah, yeah, this is there to make exert more interesting but it's also a fallow area of the game that I'm surprised WotC hasn't looked into before. Creatures tap to do almost everything, why not explore themes that reward us for using them?
The graveyard matters. Again. I'm very, very dubious about this, even though it isn't a big theme, because of what Shadows Over Innistrad had. On the upside, this could bring in some new decks, or revive old ones. On the downside, too much focus on the graveyard without any legitimate hate cards raises a red flag for me.
Cards in hand matter. Blue wants cards in hand. Red/Black don't.
You know what would thrill me? If there was a legit excellent R/B deck that came out of this. What's also interesting? This is a spot where Blue/Black don't get along.
Zombie tribal is getting a big push. I'm OK with that...up until Fuz realizes how much he can upgrade his zombie deck.
Minotaur tribal might finally become a thing, too.
Specific cards of interest:
They're going to do something with Oracle's Vault/Pyramid of the Pantheon and the brick counters. Just a matter of time. I suspect Hour of Reckoning will have some fun cards for it.
Approach of the Second Sun + Fork is a neat win condition.
I suspect Cryptic Serpent might be the new Gurmag Angler. But Slither Blade is better than it has any right to be.
Bone Picker is going to make a friend with Blood Pet. If I can create a 3/2 flying deathtouch creature on turn 1, that's cool.
Shadow of the Grave is going to create a combo deck of SOME kind.
I think Harsh Mentor might be one of the best red cards I've seen in awhile.
Manglehorn is very interesting and I wonder if it might have some impact on Vintage. I don't know Vintage very well but if it's possible to drop that on turn one, you could put a real hurt on a lot of Vintage manabases.
I think Sandstorm Convergence is going to make a splash in Commander: it's great protection and a creature generator is never a bad thing.
And that's about it. It'll be interesting to see what comes of Amonkhet. I can't say I'm thrilled by it, but I'm definitely interested by the possibilities and the interactions that might arise with older sets.
*Edited because I thought the next set was "Hour of Reckoning". My bad.