So I've been playing this quite a bit, just some first impressions here!
-As its been said, Clefable is probably the big center of the metagame. I've mostly seen Unaware Clefable, since it can stop big sweeping threats such as Gyarados and Dragonite, but Magic Guard is no joke either, since it makes a mockery of Toxic Chansey. There's also the Minimize + Cosmic Power set, which is extremely annoying to face without any way to phaze, since it can totally sweep you with Stored Power!
-Of the Psychic-types in this metagame so far, I'd say Alakazam and Starmie are the biggest threats. I feel like a lot of people aren't using Alakazam to its fullest potential though. I've just been seeing a lot of CM + 3 attacks, but the Alakazam sets that give me the most trouble by far are the ones that use Substitute with Encore or Disable, since those are less vulnerable to surprise sucker punches.
Starmie isn't as fast and powerful as Alakazam, but taking priority better has been a life saver against stuff like Dragonite, and Starmie's coverage is superb! It also gets Rapid Spin, which unlike other battle spot metagames, is very relevant here since we're playing in a 6v6 metagame. Most people use Natural Cure Starmie, which makes sense, but Analytic isnt bad either: Starmie tends to force switches out from some of the frailer stuff like Gengar, so that extra damage on the switch can be very handy! Also, watch out: Starmie DOES learn Minimize!
-Unlike RBY, I feel that having a Poison Type is actually fairly important, due to Clefable being freaking everywhere. Nidoking is the obvious choice for most teams: It can slam Clefable with Poison Jab (though this isn't a OHKO), it has valuable priority in Sucker Punch, and Sheer Force lets it use special moves well such as Earth Power, Thunderbolt, and Ice Beam. It does suffer from 4mss however.
Gengar is also excellent, despite its problems with Calm Mind Clefable. You CAN use Clear Smog as a weapon against that however, and Gengar's access to Will-O-Wisp is very valuable!
Venomoth isn't really that great at checking Clefable, but I think its an extremely under rated threat at the moment, because Sleep Powder + Quiver Dance is EXCELLENT stuff! You can even use Baton Pass over Sludge Bomb to potentially set up other sweepers! It's been making an appearance on most of the teams I've been making, because its just so damn good at using Sleep Powder.
Venusaur isn't the best at checking Clefable, but that Grass-typing comes in handy against Heat Waveless Zapdos, and of course it has Sleep Powder and an immunity to said Sleep Powder, which can be a life saver!
Some Stuff I may test out
-Curse Muk. It probably gets stopped by more Pokemon than Nidoking, but it does take special hits better, and also gets Shadow Sneak, meaning it can smack Alakzam after some a bit of damage.
-Some people have also brought up Weezing. Weezing does get Will-O-Wisp which is always handy against physical threats but the lack of physical Poison moves is gonna be a problem against Calm Mind Clefable.
-Jynx. Jynx is ultra frail on the physical side and its a bit slow, but Lovely Kiss sounds excellent against slower threats! The Stealth Rock weak is also annoying, but it can at least use Dry Skin to screw up Water-types.
