Though this is probably an incredibly overused line, Mega Metagross dominates teambuilding in OU, with it being around. If you run Offence/Balanced it forces you to carry two checks/or a bulky scizor (as I believe there are no other viable counters in those play-styles).
While it may be argued that most threats in OU require the same consideration, it's truly the lack of options for mega metagross which make it so threatening. The viability of so many moves on Metagross, plus it's massive attack stat coupled with a useful ability often leave usually incredibly good physical walls useless; ie Grass Knot vs Slowbro, Hammer Arm vs Ferrothorn, Ice Punch for Gliscor/Lando-T. Zen Headbutt severely dents Rotom-Wash.
Again, it may be argued that Metagross has 'four move slot syndrome' but it's truly the eclectic nature which is so frightening. You won't know whether your 'counter' won't get smacked on the switch in if the Metagross has the correct coverage move.
Finally, it's ability to work with various threats already in the tier, such as Keldeo, Azumarill & Talonflame as well as huge bulk & typing allows the usual player to place Metagross on their team without much preparation, as it comes with a myriad of resistances.
I wholeheartedly believe Metagross should be banned.