I found this comic recently which expresses perfectly why all-powerful characters are boring:
God Man saves the day by lifting an eyebrow. Dull, dull, dull.
No struggle, no strife, no drama, no story. All-powerful characters finish the plot far too quickly. In roleplaying games where you want to involve other characters to weave an interesting story, this is a no-no. Have you encountered a player like this in your game? Tell us in the comments below how you dealt with them.