Originally Posted by TheNightHawk3
Don't forget about Casseopia, she is banned quite a lot as well.
This is just untrue.
The common bans depend a lot of what skill level you are playing at.
At low ELO, you often see pubstompers like Tryndamere, Kassadin, Shaco and LeBlanc banned simply because unskilled players don't know how to deal with these characters and they are capable of ending a game in 20 minutes if you let them get fed. At skill high levels, people know how to not feed, and most of these characters become useless mid-late game if they aren't extremely fed.
At higher ELO, you see bans for different reasons - mainly champions that synergize well in team fights (such as Amumu, Malphite) or champions that can play around the current meta (Twisted Fate, Shen, Nidalee) by either extensive early-mid game roams and ganks or split pushing.
Beyond that, there are bans for champions that are just overpowered. Before his nerfs, Ezreal was banned quite often. Right now, I'd say Katarina is most deserving of a ban for being OP. People make the same argument for Amumu, although that's debatable.
And then there's Blitzcrank - no one wants to play against Blitzcrank. He's a champion that can easily win a game with a single pull. It's pretty reasonable to say Blitzcrank is one of the most punishing champions in the game - a single mistake will lose you the game against a team with Blitzcrank.