@Wonder
I think you're on the right track, but there's an important aspect missing.
To be honest, I think it also depends on activity of the character as well. The Higher Power is dangerous and id god-tier, but he is allowed because he doesn't actively go in and attack people with everything he has. Same for Yukari. She holds back with her powers, only using more when there's someone close to her "power level". :p
As for Takoizu, she's active, but she's balanced out, due to the enemies she fights. This means that since they can fight back, her power is okay. Now, if she attacked a human or weaker Character, the red flags would be raised in a heartbeat, if it's not for the plot.
@NeoFox
Did you reply to London?
@everyone
I feel like sharing music. Here you go. :p
http://www.youtube.com/watch?v=HqOlwKHydCk