Speed
Speed is not a simple running speed that tells how fast your character can move. Yes, it is related, but like Taijutsu is to strength, they are not necessarily the same thing. A player's speed is a combination of their hand seal ability, perception, reflexes and thought process. A character with a higher speed is able to make and act on decisions more rapidly in a battle than the other players. This means that they can create seals faster, can move and react faster and can think through situations faster so that they can act before others. Though for a Taijutsu user this will most likely mean that they are incredibly fast at moving around on the battlefield, for a Genjutsu user this may have absolutely no connection to physical ability at all. It is all determined in the style of role play you choose.