Lots of interesting stuff being revealed. They usually start off with the splashier things, so not entirely surprised there, but still, it's a good start.
Mechanics:
Cycling - returning once again. I don't see cycling cards played that often - they tend to be a bit overpriced, and the flexibility of being able to cash in a situational card isn't quite as valuable in constructed, where you can just play a non-situational card instead. Still, good stuff for
Astral Slide decks.
Keyword counters - cool mechanic, headache for bookkeeping. I can't imagine tracking these with normal counters, and I don't want to carry around punchcards with me. I don't think the mechanic is that powerful inherently (although granting deathtouch has some interesting implications), but it can be fun to pile a bunch of keywords on something.
Mutate - reminds me of Bestow. It's pretty weird, and hard to evaluate. Again, I don't think it's super powerful, but could be some interesting synergies. 'Whenever this creature mutates' triggers are super parasitic though - I don't expect we'll have enough cards available for this to be relevant in EDH, but we'll see...
Companion - this mechanic is very, very powerful - starting with an extra card you can cast from outside the game, assuming you've built your deck around it? This essentially provides a second commander, or a slightly weaker Partner (since you can't recast it from the command zone). A bit weird that they're allowing it, given that sideboards don't exist (and I'm sure this will flare up that discussion again >.>). Still, remains to be seen whether the restrictions make the extra card worth it - EDH's card pool is deep enough that it's possible to build around pretty much any restriction, but that doesn't mean a restricted deck will be fully functional.
Cards:
Vadrok, Apex of Thunder - casting a spell for free from your graveyard? This sounds perfectly fair and reasonable. Could be good played fairly to replay stuff over the course of the game, but I imagine there is some nonsense you could do. Triggering off mutation instead of ETB / casting makes it much more difficult to abuse though.
Lutri, the Spellchaser - otter! But banned. If it weren't, it would be an instant snap-include in literally every Izzet deck due to not actually having an actual deckbuilding restriction. Ignoring the Companion ability, it still would be very strong just based off the
Dualcaster Mage ability, even if it only hits your own spells.
Gyruda, Doom of Depths -
Yennet, Cryptic Sovereign's antithesis. ETB reanimation is very strong, although only hitting from the cards that were milled makes it much less consistent. Does look sweet though - note that it can hit your opponents' creatures, so you could do some stuff to put creatures on top of their owners' decks. Could be worth running in the 99.
Luminous Broodmoth - did someone say d-double d-dipping? :D Snap include in Teysa. (speaking of which, I still need to hunt down a
Nightmare Shepherd...) Card looks awesome in aristocrats/sacrifice decks. Also somewhat interesting as boardwipe protection, or if you have a way to flicker creatures to remove the counter. Don't think I'd run it in Brago (I have too many fliers), but it's intriguing.
Zilortha, Strength Incarnate -
Doran, the Siege Tower's antithesis. Only affects your creatures, but certainly interesting.
Vivien, Monsters' Advocate -
Garruk's Horde // mini-
Wild Pair? Hmmm... Tutoring creatures into play is pretty strong, as is the future sight ability. A bit expensive though - 5 mana means you usually won't get the tutor off until the following turn.
Lukka, Coppercoat Outcast - again, tutoring stuff into play is pretty strong.
Polymorph that only goes up in size is interesting - I suspect most Polymorph decks are already only running payoffs, but this makes it a bit easier to fit into random decks. Draw 3 as a +1 is also a lot, even if you do need to have Lukka out.
Illuna, Apex of Wishes - more ways to cheat stuff out? Anyway, a bit generic, but interesting.
Keruga, the Macrosage - a bit generic in payoff, but potentially a lot of card draw.
The revealed C20 commanders don't particularly excite me - they look perfectly reasonable if you want to build a
cycling /
keyword soup /
spellslinging /
human/token aggro /
mutate deck, but being three colors (and rather specialized) makes it hard to put them in an existing deck.