I don't really have a lot to contribute, seeing as most of the stuff I would've done has been done already and I'm on the fence with much of the rest, but it's still something.
Mew needs to stay banned (since it's highlighted in orange now). When a Pokemon can run any TM Move / Move Tutor move and any move from previous generations that's no longer obtainable, especially with Mew's statbase, it's a little broken IMO. This is mainly because it becomes so hard to predict, and it can set up its moves in a way so that it can become anti-metagame.
Mew can run a set with any setup move that TM's allow you to teach it, not to mention Amnesia, Nasty Plot, and Barrier. It can also Baton Pass any of those boosts, OR use them to sweep. Hell, it could even try doing both with something like NP / Baton Pass / Psycho Shock / Flamethrower or Aura Sphere or something. Baton Pass also allows it to escape from Wobbufett or Arena Trap users not mentioning Shandera with ease, and then switch to an appropriate counter.
^ That's all for just one set, and it doesn't even cover everything that Mew can potentially accomplish with that set alone. Trying to get a decent register of all of Mew's potential sets is so difficult as to become impractical.
Now, for its stats. Hm... Base 100 everything means that it can function as either a tank or a sweeper, and just about anything in between. Now, most Pokemon like that in OU that have one or two limitations that make them OU, mostly in the form of limited movepools and bad typing for either offense or defense. Mew has only one of those characteristics, the typing. It's movepool is almost limitless, meaning that it's so much harder to predict and bring in an accurate counter until you've already scouted its set. And even then, how are you going to account for everything a Mew could use?
In Ubers, Mew isn't broken because its stats are weaker than most anything else in the metagame. In OU? Not even close, Mew's stats are top-tier in OU, even for 5th Gen.
I think that's enough for now, I'll be back later to support my points if need be.