I agree with the several thousand people who have already said a teacher should be funny, and mock his pupils. My favourite teacher, Mr C (yeah I'm following the herd - I just want to be popular) is the best biology teacher I've ever had, and every lesson he calls us weird names, and pokes people with his crutches, and takes the - ahem - *mickey* out of us mercilessly. Yet he also tells us how pleased he is to see us, and that we're the best class he has and he despairs of the idiots he has to teach half the time. Therefore, we can tell straight off he's only joking and not being unfair. The use of colour is important too, he writes in green and red all the time, and all over the walls (for some reason). everyone remembers things from his class and is more interested in biology as a result.
Sometimes you like teachers for no reason. My year 9 chemistry teacher was great, but that's just because he liked burning things so much!
Also, I view teachers who mark harshly as a breath of fresh air. As a "gifted" student, it's really mindnumbing to see 'a's written over your work, with no comments on how you can improve. in year 7 i had a great teacher who kept giving me 'c's until i tried really hard on a piece of homework. that was the only time i was proud to get a b.
In answer to - I think it was Anton's - comment:
I've never seen Mr C without sweat marks all over his shirt (its gross, seriously)