Grains of Salt talked about what Cloyster hits hard predominantly and what can beat Blissey and how. I also noticed that some of his calcs are incorrect, so I'm not sure how he calced things. I'll try and fill in the other side of the issues for both Pokemon in this post as well as address what makes them tick.
After laddering for a couple days, what I've seen is not that any Pokemon is necessarily overpowered or needs a ban (granted some are much better than others, but that's too be expected), it's that not that many people neither know how to teambuild nor how to prepare for the biggest threats of Nature Power. There are many good, strong counters to both Blissey and Cloyster that don't receive as much credit as they deserve (Mega Hera, Machamp, Infernape) and the fact that people aren't utilizing these Pokemon that also happen to thrive in this meta is completely on them. With that in mind, let's address Cloyster and what can 1) outspeed and kill it and 2) what can tank hits and check it.
Outspeed/Kill Cloyster:
+1 200 Atk Mega Heracross Close Combat vs. 252 HP / 252+ Def Blissey: 644-762 (90.1 - 106.7%) -- 43.8% chance to OHKO
200 Atk Mega Heracross Bullet Seed (5 hits) vs. 0 HP / 4 Def Cloyster: 370-440 (153.5 - 182.5%) -- guaranteed OHKO
164 Atk Life Orb Infernape Close Combat vs. 252 HP / 252+ Def Blissey: 351-416 (49.1 - 58.2%) -- 63.7% chance to 2HKO after Leftovers recovery
92 SpA Life Orb Infernape Vacuum Wave vs. 0 HP / 0 SpD Cloyster: 242-283 (100.4 - 117.4%) -- guaranteed OHKO
0+ SpA Technician Mega Scizor Vacuum Wave vs. 0 HP / 0 SpD Cloyster: 252-298 (104.5 - 123.6%) -- guaranteed OHKO
252 SpA Life Orb Lucario Vacuum Wave vs. 0 HP / 0 SpD Cloyster: 291-346 (120.7 - 143.5%) -- guaranteed OHKO
252 Atk Choice Band Machamp Dynamic Punch vs. 0 HP / 4 Def Cloyster: 510-602 (211.6 - 249.7%) -- guaranteed OHKO
252 SpA Magnezone Flash Cannon vs. 0 HP / 0 SpD Cloyster: 246-289 (102 - 119.9%) -- guaranteed OHKO
252 SpA Magnezone Volt Switch vs. 0 HP / 0 SpD Cloyster: 428-506 (177.5 - 209.9%) -- guaranteed OHKO
252+ Atk Huge Power Azumarill Play Rough vs. 0 HP / 4 Def Cloyster: 186-220 (77.1 - 91.2%) -- guaranteed 2HKO
+6 252+ Atk Huge Power Azumarill Aqua Jet vs. 0 HP / 4 Def Cloyster: 165-195 (68.4 - 80.9%) -- guaranteed 2HKO
Take hits from/Revenge/Check Cloyster:
252+ Atk Choice Band Cloyster Rock Blast (5 hits) vs. 252 HP / 252+ Def Snorlax: 195-235 (37.2 - 44.8%) -- approx. 100% chance to 3HKO after Leftovers recovery
252+ Atk Choice Band Cloyster Rock Blast (5 hits) vs. 252 HP / 232 Def Mega Slowbro: 150-180 (38 - 45.6%) -- guaranteed 3HKO
252+ Atk Choice Band Cloyster Rock Blast (5 hits) vs. 252 HP / 252+ Def Suicune: 190-225 (47 - 55.6%) -- approx. 7.4% chance to 2HKO after Leftovers recovery
252+ Atk Choice Band Cloyster Rock Blast (5 hits) vs. 248 HP / 252+ Def Mega Scizor: 170-200 (49.5 - 58.3%) -- approx. 99.6% chance to 2HKO
252+ Atk Choice Band Cloyster Icicle Spear (5 hits) vs. 240 HP / 0 Def Eviolite Doublade: 105-120 (32.9 - 37.6%) -- approx. 99.3% chance to 3HKO
252+ Atk Choice Band Cloyster Icicle Spear (5 hits) vs. 252 HP / 16+ Def Mega Aggron: 145-175 (42.1 - 50.8%) -- approx. 0.4% chance to 2HKO
(Atk and Def Switched)
252+ Atk Choice Band Cloyster Rock Blast (5 hits) vs. 252 HP / 252+ Def Vaporeon: 195-235 (42 - 50.6%) -- guaranteed 3HKO after Leftovers recovery
252+ Atk Choice Band Cloyster Icicle Spear (5 hits) vs. 0 HP / 4 Def Mega Metagross: 195-235 (64.7 - 78%) -- guaranteed 2HKO
252+ Atk Cloyster Icicle Spear (5 hits) vs. 56 HP / 0 Def Mega Heracross: 255-305 (80.9 - 96.8%) -- guaranteed 2HKO
252+ Atk Choice Band Cloyster Ice Shard vs. 56 HP / 0 Def Mega Heracross: 123-145 (39 - 46%) -- guaranteed 3HKO
252+ Atk Cloyster Rock Blast (5 hits) vs. 0 HP / 0 Def Infernape: 255-305 (87 - 104%) -- approx. 6.3% chance to OHKO
Realistically, your team doesn't even need a Check to Cloyster, as long as you have a Pokemon that can outspeed and kill it (anything faster than Adamant base 70s). And bare in mind, the important fact that Shell Smash isn't the most viable set for Cloyster (not sure about usage, but Band is simply better), so you will most likely be able to outspeed it. And, if Shell Smash does become/is an issue, we could always just suspect Shell Smash (though I really don't see that happening). Not only that, but the choiced sets are counter-able by just about any priority check and by following what they've locked themselves into.
Moving on, I've noticed that this point hasn't been made yet from what I see, but in case you forgot Cloyster is weak to Rocks and lead rockers are
extremely common in this tier (Shuckle, Ferrothorn, etc.). So unless you give your opponent a free defog/spin or sack your rocker, you will already have a counter measure in place. Cloyster also doesn't have a diverse moveset (Explosion, Spear, Blast, Shard), it can really only swap out a move or two from the generic set, so it isn't like you're going to face something you can't be ready for. And don't blame me if you face that one guy that runs a special lure Cloyster with Specs Surf or Ice Beam. Sure, Cloyster is a huge threat in this tier (what wouldn't be with access to a 180 attack stat), but it most certainly is beatable and not by any means broken.
NO BAN for Cloyster.
I will add my thoughts on Chansey later when I have time to continue...