Alright: we've got the official spoiler list here, what I feel is a readable list here, and the mechanics article, in case you haven't seen that.
No sense in burying the lede here; so far I'm a fan of Dragonstorm! I think there's some neat cards, I really like how the mechanics create identity and distinction for the clans of Tarkir, and I think the execution of the shard aspect of the set is handled well.
There's a couple silly cards but maybe they aren't as bad as I think? Let's talk.
White
One important note is that while Clarion Conqueror is a damn good body at the price, it shuts down everything, including your stuff. Important to keep that in mind during pre-release weekend. But I haven't seen an ability like that in awhile, so I'm glad it's there.
The Devotees-we'll start with Mardu since it's first-are a VERY smart way to help fix colors in this set. I feel like these are smart design choices that will help people enjoy Dragonstorm and I was very glad to see each one.
The Flurry ability reminds me that this set looks to be fairly mana hungry-at least in Limited. I'm already wondering if 17 mana sources is correct. But in constructed, I just want to start looking for any spell I can play without paying mana to try and maximize cards like Sage of the Skies as early as possible. That could easily come out turn one with a Lotus Petal, City of Traitors start, and having two of those that ear? YIKES.
United Battlefront seems weird and is probably bad, but I want to mess around with it anyway.
Blue
The Renew ability on Agent of Kotis just reinforces what I was saying about Flurry: having enough mana in your games is probably going to be a big deal. Assuming there's a payoff, anyway. But getting to four mana and then deciding that you want to spend it putting +1 counters on a creature isn't a bad decision, just one that you don't want to blow your tempo on too much.
Similarly, a card like Bewildering Blizzard feels like it could be backbreaking--but you still have to get to six mana to play it.
Did Winternight Stories need to be a rare? I feel like we've gotten other effects like this at uncommon and they've been fine. What's the upgrade here? It doesn't seem THAT much better.
Black
Avenger of the Fallen looks bonkers. That is all.
I also really like the design and feel of Hundred-Battle Veteran.
There's a new legendary enchantment, which is wild! I haven't seen one of those (The Sibig Ceremony) in a long time.
Red
Channeled Dragonfire is making me rethink whether or not Harmonize is a good ability. Seven mana to flashback two damage is bad and it doesn't matter how you slice it. Reducing the cost to reasonable by tapping a five power creature? How is THAT good? Firebolt came out in Odyssey and it was fine but nobody was going to mistake it for a format defining card.
It may be that Harmonize has a similar issue that Vehicles in Aetherdrift had. But we shall see.
Dracogensis is a stupid card and I hate it. It isn't interesting, it allows for very stupid things to happen, and practically screams "Glory of Cool Things". It's particularly insufferable for Commander, the format it's obviously for.
Green
I'm very happy to see a Craterhoof reprint; the card is too expensive and a lot of fun.
Is Dragon Sniper one of the best one drops ever? Certainly one of the best defensive ones and I bet will be an all star in the Limited format due to all the flying creatures.
Multicolor
Here's where the money is, right? Auroral Procession feels like a color pie shift: I would expect this as a G/B card, not G/U. Perhaps this is allowed because of the philosophy of the color wedge in this set?
I think all of the Sieges are flavorful and interesting, really like those choices.
Call the Spirit Dragon; this card is stupid and I hate it. It even says "win the game" on it, but then it makes you jump through hoops in order to do so. This actually makes it worse, since making all your Dragons indestructible should already win you the game. Another card that is particularly insufferable for Commander, the format it's obviously for. It's unfun, and scuttles interaction.
Digging a little further; there's a lot of very cool things that start around the five mana slot. Again, making me think that the Limited environment might reward players who plan for a late game and have the mana for it.
Artifact/Land
Ugin on its face reminds me of Sire of Seven Deaths; if you can cast that, you win. And on one hand: OK, big mana spells should do big things! But on the other, if you've ever played against a Sire of Seven Deaths you know how unfun that is. Crushing to any gamestate it enters into, it flips the game around in ways that don't allow it to be unflipped.
I'm not a huge fan of these designs, but Ugin might be slower and more interactive. We shall see.
The rest of the artifacts are solid: good role players to help us do cool things in the format. I'm not upset about any of it: Mox Jasper will be broken as soon as someone can utilize a (changeling) Shapeshifter with it, but...I'm actually OK with this. It's conditional and most shapeshifters don't have other abilities that keep players from interacting.
The common and uncommon lands are known quantities, good mana fixers. No notes.
The rare cycle of lands, these are cool! They are mostly great mana fixers I think: the abilities on them are neat but not broken by any means. Still; don't sleep on them, they have great utility.
All in all, I like this set and I'm eager to try it out! Will the Omens be as interesting as they appear? Does this set need good mana planning in Limited? How important will Reach be?
I'm interested in finding out. Let's go!