Arise dead topic! The thread necromancer approaches!
For me, a CS should give the GM everything they need to help guide that character through the story, and ensure that all the characters are diverse. If everyone's characters are mercenaries, mine will usually be militia. If everyone's bloodthirsty killers, I'm going to be a saint.
I think some core things that should be on every CS are the name, age, sex, occupation, and skills the character possesses. If combat is a focus, knowing their weapons and fighting styles is always a plus. Appearance isn't necessary, hopefully you'll give a decent description of them when they appear in your first post.
The bio field is probably the most misunderstood part of the entire CS. At least for me it is. It's important, but how big or how small should it be? Do I need to include my character's upbringing by wolves? The loss of a loved one? What about particulars I'm unsure about? I like to reveal the story behind my characters in the RP itself, but obviously it is nessisary to give the GM an idea of what drives your character. I usually choose to settle on what most people should generally know about the character, keeping more personal details covered until the opportunity to reveal them shows itself.