Having a "Brief History" and "Background" section in your CS. They are the same thing. Just because you want one to be long and wordy doesn't mean it's not irrelevant. 
(I said it'd be fluff and redundant, in more words, and I was told that's not how I should go about presenting my CS)

I'm sure this has already been said: Personality and Full History in a CS. These are pretty self-explanatory...maybe >.>
I'm sure this has also been said: "The CS proves your writing capabilities." It does not and it's just a formality to not have a Mary Sue or OP'd character and to make sure there is a general understanding of the plot and setting. I feel that's the only reason for a CS.