Ok, I've been thinking, and the other kick-ass, mysterious teacher besides Kakashi, but who is rarely discussed, is Asuma. There's lots we don't know about him:
1) His name is Sarutobi Asuma. The 3rd Hokage's name is Sarutobi. (However, we don't know if that's his first or last name. Orochimaru refers to him as Sarutobi-sensei, which makes me want to think it's his first name, since all the other genins call their senseis by their first name, i.e. Kakashi-sensei, Gai-sensei.) Is he the 3rd's son? Would that necessarily make him Konohamaru's father? We know that the 3rd had to have at least one child, since Konohamaru is revealed in canon as his grandson.
2) What special skills does he have? Is he another taijutsu specialist like Gai? I tend to think not, but the only thing we have seen him to is fight with his brass-knuckles/dagger thing. (Anybody know the term for this?) My feeling is that he can do at least some ninjutsu, as he can do kawarimi and the fact that ninjas that cannot do at least some ninjutsu and genjutsu are very rare.
Let's hear somebody else's opinions!