I'm actually not a tough critic myself, I'm more the kind of teacher that says "Ok your timing was good, now if you would add <insert detail> it would work even better". Or if I correct someone on a point of etiquette I try to give a reason for the correction.
Hmmm...I take that back to some extent. One thing I think it's important to do is to check if people understood a correction and actually are able to perform it. So I usually stick around a watch for a while and if things don't improve I might remind people a couple of times. That does feel difficult sometimes and make me wonder how annoying it is.
But however carefully you word things, you just can't always know how people will interpret them. I remember once a guy sighed after a class that I'd lead that he really sucked more than usual today. Thing is, he'd been doing really well so I'd given him loads of corrections during the class...