This is a little late, but I think this is important to clarify.Couldn't access my old account. Turns out hotmail wiped out my previous e-mail so no more 2K+ post count for me.
Anyway, voting for ban. Noting beforehand so I can avoid obvious calcs. Yes, this thing can be easily checked.
Things people think counter but really don't
mSlowbro
0 SpA Tough Claws Mega Metagross Grass Knot (100 BP) vs. 252 HP / 0 SpD Mega Slowbro: 240-284 (60.9 - 72%) -- guaranteed 2HKO
mSableye
252 Atk Tough Claws Mega Metagross Meteor Mash vs. 252 HP / 252+ Def Mega Sableye: 132-156 (43.4 - 51.3%) -- 98.8% chance to 2HKO after Stealth Rock
0 SpA Tough Claws Mega Metagross Grass Knot (120 BP) vs. 0 HP / 0 SpD Mega Gyarados: 192-226 (58 - 68.2%) -- guaranteed 2HKO after Stealth Rock
252 Atk Tough Claws Mega Metagross Zen Headbutt vs. 248 HP / 216+ Def Rotom-W: 133-157 (43.8 - 51.8%) -- 69.5% chance to 2HKO after Stealth Rock and Leftovers recovery
0 SpA Tough Claws Mega Metagross Grass Knot (80 BP) vs. 252 HP / 24 SpD Slowbro: 186-220 (47.2 - 55.8%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
Things in OU that do counter
84 Atk Mega Scizor Knock Off vs. 0 HP / 0- Def Mega Metagross: 112-132 (37.2 - 43.8%) -- guaranteed 3HKO(assuming they stay in)
252 SpA Rotom-H Overheat vs. 0 HP / 4 SpD Mega Metagross: 338-398 (112.2 - 132.2%) -- guaranteed OHKO
Only two counters currently that don't become dead weight on the team. I've been involved in the metagame since the days of Shoddy. Relying on Burn, Toxic, and something like Whirlwind to save your team from something as unhealthy as this is poisonous to the metagame. ToxicStall and BurnStall should never be an option for countering something. Yes, this thing can be checked but then again, so was Aegislash, Greninja, mKangaskhan, mMawile, and so on so forth honestly. The check argument gets old real fast. I would love for this thing to stay in the metagame but sadly I don't think this can happen.
Does mMetagross benefit the metagame? To a degree, yes. It can check many Fairies and counter others as well. However, Heatran, Scizor, Magnezone, and such do the same as well. Does this Pokemon prevent others from being used? As much as other Pokemon in the current tier do to each other, yes. That is the natural order of things if I recall correctly. As said previously, mMetagross pretty much gives us the Greninja effect all over again. You don't know what set it's running. It can run over your team either way. Voting ban honestly.
EDIT: Noticed BB code has been done away with. The hell.
EDIT: For the sake of ruining the arguments of others
252 Atk Tough Claws Mega Metagross Ice Punch vs. 252 HP / 56+ Def Gliscor: 340-404 (96 - 114.1%) -- guaranteed OHKO after Stealth Rock
252 Atk Tough Claws Mega Metagross Thunder Punch vs. 252 HP / 252+ Def Skarmory: 136-160 (40.7 - 47.9%) -- 3.9% chance to 2HKO after Stealth Rock and Leftovers recovery
I honestly don't even know why I bothered listing Skarmory. It can't damage mMetagross to save its life.
0 SpA Tough Claws Mega Metagross Grass Knot (100 BP) vs. 252 HP / 0 SpD Suicune: 178-210 (44 - 51.9%) -- 69.5% chance to 2HKO after Stealth Rock and Leftovers recovery
-1 252 Atk Tough Claws Mega Metagross Ice Punch vs. 252 HP / 240+ Def Landorus-T: 248-292 (64.9 - 76.4%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
Shame Landorus-T gets outsped, eh?
252 Atk Mega Metagross Earthquake vs. 4 HP / 0 Def Bisharp: 238-280 (87.5 - 102.9%) -- 56.3% chance to OHKO after Stealth Rock
252 Atk Tough Claws Mega Metagross Meteor Mash vs. 248 HP / 252+ Def Tyranitar: 288-342 (71.4 - 84.8%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
252 Atk Tough Claws Mega Metagross Hammer Arm vs. 252 HP / 88+ Def Ferrothorn: 212-250 (60.2 - 71%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
SpA Tough Claws Mega Metagross Grass Knot (120 BP) vs. 252 HP / 0 SpD Hippowdon: 314-370 (74.7 - 88%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
0 SpA Tough Claws Mega Metagross Grass Knot (80 BP) vs. 4 HP / 0 SpD Starmie: 182-216 (69.4 - 82.4%) -- guaranteed 2HKO after Stealth Rock
I get your point, but you missed some seriously important points. Mainly missed some EVs/ most likely situations.
For example, megabro isn't going to switch in unless it KNOWS you aren't running gk; it's not like it's exactly a novelty move at this point. Megagross is going to have to switch in, probably at +1. Suddenly the tables have turned against Megagross. Also, I am pretty sure no one runs 0 SpD on Megabro, just saying.
Next problem: Gyarados, if smart, won't mega evolve immediately to avoid getting hit by a SE gk or hammer arm. The only move that could hit it for SE damage would be thunderpunch, but Megagross is at -1 attack. Gyarados uses DD, THEN mega evolves, outspeeds, and ohko's w/ crunch.
It's a long post with a lot of calcs that i don't want to go through and rip apart, but you get the idea. Megagross is better on paper than in reality.