Well, looks like I got to the party when it's going full steam.
I think that
Mega Kangaskhan should not be banned.
I have personally made at least 5 teams around this guy in Pokebank, and Mega Kanga far from guarantees a win. It operates similar to any other sweeper, and has just as many checks. Volcarona has Heatran, M-Kanga has Keldeo. Keldeo resits its only source of priority, and outspeeds and OHKOs it. All you have to do to stop a M-Kanga sweep is put in a Keldeo, and go for the Sacred Sword. STAB+ Super Effective will do Kanga in.
For non-anecdotal evidence, let me reference the last ban debate. Gengarite was banned for its capability to offer next to perfect support, without anything able to effectively oppose it. As the metagame has evolved up to this point, it seems to be leaning strongly towards physical defense. Ferrothorn, Skarmory, Forretress, and others are becoming more and more prevalent than ever, while special cannons like Latios and special walls like Blissey are being substituted for things that can handle physical hits as well, like Heatran. This is due in no small part to the physical threats of this generation, like Talonflame, M-Luke, Char-X, M-Kanga, Aegislash, and others. This means that removing a physical threat like Kanga will not fix or alter the flow of the meta. It will simply be one less threat for teams to deal with, instead of something that can help shape the meta due to how teams prepare for it.
In reference to the "Portrait of an Uber" thread, this is what is listed as the qualities for an offensive Uber:
Offensive Characteristic
A Pokémon is uber if, in common battle conditions, it is capable of sweeping through a significant portion of teams in the metagame with little effort.
Kanga has the "capable of sweeping through a signifigant portion of teams" part down pat, and that is where all of the adversion to it seems to be coming from. However, it is unable to follow through on the integral "little effort" section. As with most sweepers, certian threats on the opposing team must be dealt with before attempting a sweep. For Volcarona, it is Stealth Rocks, Rock Types, and Heatran. For Dragonite, it's status setters, Faries, Ice Shard carriers, and things like Vaporeon. For Aegislash, it's WoW carriers, Normal types, and faster mons with Earthquake. You get the point. For Kanga, the list is no shorter. Before you can get to sweeping, you need to get rid of the opponent's Sableyes, Quagsires, Ferrothorns, Rocky Helmet (and Roar) Skarmory, M-Lucario, Fighting types with priority (Read Mach Punch), and Ghost types. If there is a Ghost type still alive, Kanga
CANNOT sweep (assuming it is played well). Many ghosts carry WoW, which cripples Kanga without PuP, and ones with it cannot use it on a ghost type. Jellicent, Sableye, Dusclops can all do this well. Then, their only fear is a Shadow Claw, but if Kanga is carrying that, it allows Ferrothorn to take it to meet its maker. If the opposing team is running any good form of physical defense on their team, a large ammount of effort is required for Kanga to sweep, which, by definition, makes it not Ubers.
Kanga is also very susceptible to passive damage. With Drain Punch as it's only form of effective (offensive) recovery, it has a hard time staying the battle unless it can pull of a sweep. Priority, status (Esp. Burn), Stealth Rocks, and Spikes all keep Kanga's number of switch-ins limited. If it chooses to run Drain Punch it must give up one of the things that makes it such a potent sweeper: Priority, Strong STAB, Boosting, or Coverage. "But what about Wish/Seismic Toss?" By definition, that is not a sweeper. It hits hard, yes. However, it does not have the sweeping prowess of normal M-Kanga. Seismic Toss is also reasonably gimmicky, since it (again) cannot deal with Ghosts, anything with a reasonable amount of bulk that runs substitute (and let's be honest; most things with a large ammount of HP bulk love to run sub). It also has to 2HKO, since almost nothing in OU has less than 200 HP. This allows opponents to switch in Ghosts, Recover, or hit Kanga with status.
In closing, Kanga is good, but it is nothing new. I would support suspect testing it, but under no circumstances should it be insta-banned. It does not have the ability to sweep the Meta unopposed, and therefore does not fit the qualification for an Uber. It is not uncounterable. If it wants to live long enough to take advantage of its bulk, it has to diminish its sweeping capabilities. If it goes defensive, it becomes easier to counter, and only succeeds in changing its counters, not removing them.