Okay, I WAS wrong, Registeel's OU Tank set cannot be 2KO'd by a Skymin's Earth Power + Seed Flare, however, that still doesn't change what I've said already. Note that I'm not taking any sides, as with Garchomp, I really don't care about its status.
Reopening old wounds, I'm going back into Garchomp. Garchomp caused the metagame to overcentralize on itself, only a few counters existed for it and they all had a chance of being weakened to the point where that Garchomp could KO them all. Registeel and Regice well somewhat countering Skymin just as Mesprit and Bronzong when Garchomp was around. So, Skymin doesn't get a move to boost its Special Attack two stages? Seed Flare from a base 125 Special Attack with an 80% of a -2 Special Defense drop can cause even the bulkiest of walls to think twice before switching in. Tell me this :/, if you were running Skymin and you know damn well that Registeel and Regice could be running around, wouldn't you carry something to kill those off? If you have Stealth Rock up, Scizor can either setup a secondary sweep, or, do some damage and Baton Pass a Substitute to Skymin, allowing it to attack safely while getting rid of its "counter".
Any good player will attempt to remove their core pokemon's counter, so this argument is irrelevant. Am I going to try to SubCM Raikou sweep knowing that the opponent has Blissey? No, I'm going to get rid of it with something like Dugtrio or a physical pokemon.
Basicly, in the right hands, Skymin can sweep whole teams just as Garchomp could be if it gets up a Swords Dance with a Yache Berry and like Garchomp there's at least a 95% chance that you'll lose a Pokemon to it if your opponent knows how to use it.
Wrong again. Shaymin has base 120 sp.atk, 10 points lower than Garchomp's attack, it has worse defenses in all aspects, and SR + Sand prevent it from taking full advantage of a Yache Berry. Besides that, if it gives up Life Orb, then it isn't hitting nearly as hard and MUST use a Growth set to deal as much damage, thus sacrificing coverage.
Now, you might say "Well, any Pokemon can be dangerous if someone can use it right". That is correct, however, you need to remove ALL of its counters from play before it can sweep. Dragon Dance Tyranitar for example can outrun a lot of speedier Pokemon and double its already monsterous attack after 2 Dragon Dances and it gets Taunt to mess up anything that tries to cripple it. Now, let's look into its counters... Donphan, Rhyperior and Hippowdon are good counters for the ones lacking Ice Beam and Swampert can counter Ice Beam versions if they lack Hidden Power Grass. Bronzong is an all around counter for it. There are more sets and more counters. That's a lot to look out for, isn't it?
Skymin has only Regice and Registeel as exact counters and eventually, someone is going to utalize Quick Attack-Yache Berry as a way to take out Weavile leads if that is Skymin will ever lead >.>. Registeel has a lot of things that stop it, Will-o-Wisp, Reflect and most Ground moves will eventually wear it down to 45% which is the minumum for a Life Orb'd Earth Power to KO the standard OU tank set. Regice has a weakness to Stealth Rock which will almost always be used with Skymin and its it's Poisoned, or, hit with a Fighting Move from the likes of Heracross, Infernape, Hitmontop (yes, Hitmontop), etc.
Exact counters flew out the window since Diamond and Pearl were released. SD Flame Orb Guts Heracross has like no counters, but it can be worn down very quickly with passive damage, and the same can be said of Shaymin. Regice only risks being 2HKOd by Seed Flare if it chooses to go Bold and no Sp.Def EVs. Throw in some EVs, like we have done with Blissey, and it regains its ability to wall it effectively. What you are saying here is that basically, Skymin's counters can be worn down so Skymin can sweep. This can be done for any other sweeper as well.
Now, onto its actual movepool-
Air Slash- A STAB, 75 BP Flying move with a 60% chance of a flinch, basicly making ScarfKiss and ParaHaxKiss cry due to its 127 Base Speed, slight ability to take hits and a base 125 Special Attack to go from.
Again, base 120 sp.atk. Also, 75 defenses are not really that impressive, 100 HP is all that saves it - and that can be revoked by Stealth Rock.
Seed Flare- A STAB, base 120 Grass Move with an 80% chance of a -2 Special Defense drop. That's nearly idential to using Nasty Plot. There's only a 15% chance of a miss, so, if I'm doing this right, Seed Flare has a 15% chance of missing and a 20% chance of not lowering the Special Defense of the switch in, so, overall, Shaymin has a 65% chance of KOing a bulky wall with Seed Flare after it switches in. Like those odds, eh? Well, consider that there's a 120 BP, STAB Grass move (which has good coverage) coming at you from a 125 base Special Attack, won't that dent something... hard?
Seed Flare is great, but it will only get you so far. Blissey, Registeel, smart switching, etc.
Psychic- Skymin's secondary option of taking out Gengar, however, with Seed Flare + Air Slash already ruining its switch in, Psychic is only worth considering on a full Specs set without Hidden Power / Leaf Storm.
Even on a Specs set it is a poor choice. Psychic has awful coverage and simply provides Skymin's counters with more opportunities to switch in.
Earth Power- This would've screwed over nearly any arguemnt against Skymin since now, Metagross AND Heatran can't counter it fully. Earth Power takes out Fire, Steel and nearly anything that resists Seed Flare and Air Slash.
It doesn't, however, hit Bronzong, who can take its special hits quite well.
Substitute and Leech Seed- Ugh, faster than Sceptile and a bit more bulkier and powerful. Sceptile can only run a Swords Dance set better and then you realize that Skymin gets a Physical movepool as well as Swords Dance :/.
Subseed will lack the power of purely offensive Skymin, so handling it should be even easier.
Hidden Power- Coverage at its fullest, Skymin can now hit what everything else resists.
Hidden Power's only purpose is for coverage anyway, so this is not unique to Skymin.
101 Subs to SubSeed Blissey with, Quick Attack to function as an lead-anti with a Yache Berry and of course, a large base Speed and Special Attack. Not saying it should be Uber for this, but, damn, that's a lot.
Just my two cents, many things will need to overcentralize their own usual EVs, movesets, etc. just to keep it at bay.
EDIT: Another thing, typing :/. Shymin has a neutrality to Electric, resistance to Fighting and Water while being immune to Ground, that blocks the all common Surf, Thunderbolts, Waterfall, Earthquake, Earth Power, etc.
Notice that several of those attacks are paired with Rock / Ice attacks, so it still need to be wary when switching in.