Evaluating new cards and new mechanics is hard. It's also one of the best ways to get an edge early on in a Limited format. At the beginning of Guilds of Ravnica Draft, Experimental Frenzy would go around the whole table. Turned out that card was one of the best cards in the entire set. Thoughtbound Phantasm would go late. Sonic Assault wasn't considered playable. Same with Wishcoin Crab.
As the format developed, all these cards became very important in specific archetypes. It can be difficult to properly evaluate a card like Wishcoin Crab without the full context of the set. It turned out that there were enough two-toughness creatures that the Crab really made combat difficult and that there were not many ways to get through a five-toughness blocker. While we haven't seen enough of the set - especially the commons - to evaluate cards within the context of the environment, we can start the process of evaluation with what the guild mechanics mean for the format and how they interact with each other.
I believe riot will be a format-warping mechanic. This set is going to have more creatures with haste than maybe any format we've seen. 60% of archetypes (technically more thanks to the Gate deck) will have access to haste creatures at low rarities. Keep this in mind when playing a game. If you're at two life and in a topdeck war, you may not be able to attack because the probability of a haste creature is high. This isn't usually the case, and I expect it to drastically impact archetypal design.
Frenzied Arnyx is quite the common. A 4/4 trample for four with a relevant activated ability will almost always be good. And if you need to push damage, the 3/3 haste mode will end the game. I expect Gruul to be filled with cards like this. With Zhur-Taa Goblin, it's possible to curve haste creature into haste creature into haste creature. I think you get the point; just be careful when trying to race Gruul.
I also expect the existence of riot to mean you can never choose to draw in this format. The play-draw decision comes up in every format, and while it's often correct to choose to play, there are plenty of exceptions to the rule. If you have a bunch of early plays like Dead Weight and Thoughtbound Phantasm in your Dimir deck, feel free to choose to draw. Haste creatures on the play can put your opponents so far behind. One of the reasons that Dead Weight lets you choose to draw is that it's very cheap interaction where you usually end up on mana and don't take damage from the card. Well, a haste creature is going to get in for at least one hit. Maybe there will be multiple cards like Shock to mitigate this, but we tend not to get cheap, instant speed removal anymore.
Addendum is a hard mechanic to evaluate without playing with it. We haven't even seen many cards at lower rarity yet with the mechanic. The important thing to understand is that the set is designed with this in mind. I expect Azorius to have access to creatures with flash and many instants without addendum, such as Deploy.
The dance between addendum and riot may be a key to understanding the format. With the density of potential haste creatures, casting a card at sorcery speed for an upside has additional risk. I imagine Sphinx's Insight will be one of the easier cards to play with. If you don't have something else to do with your mana, cast it at sorcery speed. If you can interact, then hold up interaction. Two life is not the bonus I'm looking for, but the card is certainly still playable. Other cards with a bit more upside can cause a real headache to play with given all the haste creatures running around.
Hopefully there will be more to say about this mechanic once we see more cards.
Adapt is pretty much just a more flexible take on monstrous from Theros. I don't know if we will get Bioshift, but be on the lookout for ways to remove counters--like Galloping Lizrog--to gain maximum value from adapt.
Wow is Aeromunculus a solid common. A 2/3 flier for three is a fantastic rate, and the ability to turn it into a 3/4 at instant speed is nothing to scoff at. If we see more cards like this, Simic is going to be quite the archetype. There are already a good chunk of ways to manipulate counters and the more the merrier. Biogenic Upgrade can create three huge monsters if you've adapted, Titanic Brawl could just be fantastic if it's consistently one mana, and there's even a card like Weaver of Currents [ Editor's Note: Card's name is unofficial as of this publishing.] to help spend all this mana on adapt.
Furthermore, adapt perfectly synergizes with both addendum and riot. Well, the mechanic doesn't really synergize, but the way the Limited environment is designed will. Addendum means that there will be plenty of cards to cast at instant speed, which makes activated abilities like adapt much better. And riot creates a larger density of creatures with +1/+1 counters to synergize here. I imagine that Bolrac-Clan Crusher will be quite the card in Temur.
Spectacle is another mechanic that's hard to comprehend without full context. Will there be lots of evasive early creatures in black and red? Will there be many ways to guarantee loss of life even if attacks aren't good? Looking at the cards we've seen so far, this will be very important. Many are quite bad if you're casting them for their normal cost but very good for their spectacle cost.
Rakdos Roustabout may be the linchpin of the Rakdos archetype. A 3/2 for three is below rate, but turning on spectacle whenever it attacks is so important. A problem I see with this mechanic is that if you're banking on combat to facilitate spectacle, you're giving your opponent too much agency. If your opponent kills your random three-drop, that's fine, and the three power means it should trade in combat (although it will trade down a reasonable portion of the time). However, given how cheap spectacle costs look, it may provide a pretty big mana/tempo boost.
Another thing to look for that may contribute to the success of Rakdos is cheap creatures with riot. A haste creature greatly increases the probability you can do damage in the combat step. Imagine a turn where you play a 2/2 haste,
Afterlife gives riot a run for its money for the best Limited mechanic in the set. A 1/1 flier isn't worth a full card in most Limited formats, but it's not that far off. Afterlife 1 tacked onto any random body is likely enough to make it solid in Limited. And God forbid a reasonable creature has afterlife 2 or 3. More importantly, I imagine that this makes cards that clean up excess one-toughness creatures more reasonable inclusions. Especially Cry of the Carnarium, as that won't even let your opponent trigger afterlife.
The vigilance on Imperious Oligarch is unassuming but a key aspect to why the card is so powerful. Afterlife is a mechanic that naturally works better on blocks. Your opponent can always choose to take the damage and not trade with the card, but they don't have as much luxury here when they're the one attacking. Vigilance on this two-drop makes it the perfect curve creature in any Orzhov deck.
Additionally, ways to sacrifice creatures become powerful in Orzhov. A 1/1 flier isn't worth much at the beginning of a game, but once your opponent is at a lower life total, those little critters get the job done. I imagine Pitiless Pontiff won't be the only sacrifice outlet and that they will play important roles. This may also be the key to success in Mardu since 1/1 fliers are so good at turning on spectacle!