I understand, I was being purely visual in my list.
When it comes to character development itself, well, it's "what fits well with everything else". A character can have a long difficult history, but that's just silly if they are in a comedic manga, in which case their personalities and quirks are important.
If it's a serious anime based on character struggle, then obviously a character's depth is crucial.
Much like visuals, what makes a good character background is subjective. You could love a good backstory or view it as un-neccesary and overly contrived.
Sometimes a character background serves to flesh out a concept, sometimes to be a backbone of a story arc. Either way it's all up to interpretation.
Personally, I am fond of a character with a mixture of both bizarre quirks and a realistic issue that relates to normal life.