Well, Mega mawile hits like a speeding wrecking ball, and only two pokemon can safely switch in and check the demonic beast, and they go by Rotom-H and Heatran. Seriously, if you don't have one of those two, Mega Mawile can go on a rampage, and it's not pretty. I'm still unsure of a ban or not since little to no checks =/= ban, but odds are it'll be banned anyway.
Three Pokemon, actually. Rotom-H, Heatran, and the monster known as based pinfish.
Qwilfish aside, Mega Mawile is pretty hard to deal with due to having two very viable but vastly varied sets. And not everyone is just gonna call up Qwilfish to deal wiht it. I mean they could, but they won't. So Mega Mawile unfortunately looks pretty ban-worthy. Damn OU, why can't you be stable?