Stalling power for long time.
Forgetting to eco or doing it in wrong way. Ringing t1 mexes and not ringing t2-t3 ones.
Being very polite. Much more than high rated players. Thanking for e. Thanking for units. Thanking for help on battlefield. Thanking in many words for teaching the game, in the end. etc.
Never complaining. Never accusing or insulting. Never of course is bad word, but I don't remember ever seen someone new to game complaining, it usually comes much later.
Very open for teamwork. Tell him to build eco, he builds eco. Tell him to build t3 air, he builds t3 air. Tell them to push all together in south, they push all together in south. If they don't know, they (sometimes) ask and discuss to learn.