I have seen a LOT of lists on the MobaFire Blog recently, and I would like to point out some of the reasons I discredit them.
Mains
The way I see people choosing Main Champions is "I remember getting really fed this one game and I've played him like 13 times, so I have mastered him.". I personally feel that if you "Main" a champion or claim to have mastered a champion, you should have played at least 100 games with that one champion. That way, you know how to build this champion in every situation, how to play that champion in every lane, in every role, etc.

Allow me to show you my Mains:
  • N/A
  • N/A
  • N/A
Now this is just me. I don't have mains. I know some other people have no doubt gotten 100+ games with a champion, but honestly, I don't even think I have gotten close to that. Maybe ~50 games as Gangplank, but that is it. I just don't play the same champions too often. I'm the kind of guy who waits until everyone says what role they want to play, and then I pick the last one. (Usually Mid or Support)

BTW: You cannot say you main Varus because that is just ridiculous. He's way too new for someone to have figured out all the ways to play and build him, regardless of how cookie cutter the DPS builds can be.
Over Powered Champions
A pet peeve of mine is people always claiming certain champs to be really over powered. I think I will just make a list of champion who can be misconstrued as over powered but really aren't.
  • Xin Zhao- He is just too horrible late game, because he has to get "In the enemy's face" to deal any damage, and any team can just focus him down to nothing.
  • Tryndamere- Kite this sucker and there's nothing he can do. Easy to kill with even a moderate amount of cc and minimal team coordination.
  • Sion- Just don't let him get farm, bro.
  • Veigar- He is soooo weak early game. Your team should capitalize on that. Yes, he is a monster late game. QSS should be able to stop his stun in a teamfight, so look out for that. And again, Just don't let him farm, bro.
  • Shaco- First of all, AP Shaco is a joke. Second of all, while he may get fed early game, he doesn't have a huge presence late game. Kind of like Twitch and Evelynn, but not as bad.
  • Master Yi- You're joking, right?

Now, here are my list of OP champions: If anyone wants to share thoughts or wants an explanation to the above, feel free to comment and give me some feedback.