Commenting on your opponent's move during the game
Passing judgement on your opponent's move during a game. Or giving move suggestions during the game.
DeathWind: It's better to altogether not comment anything during an ongoing game. Complimenting a move is okay to some extent. Even though I don't mind a compliment of this kind, I realised one thing during my games with my pro teachers. They have never commented anything at all during a game on my moves. All those should be done after the game at the review itself. This is something I am striving to adopt.
Some people might not like being suggested moves in a game.
To put it short, don't comment during playing unless you have a very good reason for doing so.
kb: This is an opinion, not fact. I might agree with you here, but 'it's better to ...' is very strong language. What about social games? Teaching games?
Tas: It seems most agree that even in teaching games you should wait till after the game.
But if it is played over the board and not on the net then you might not remember it. In casual games I often discuss sequences with my opponent, shortly after they occur, so that we both might learn.
Tarka: Whenever I ask for a teaching game, I actively expect to be spoken to constantly - isn't that the point? Anyone can (and should) ask a higher ranked player "what did you think of my game?" after the game has finished, but a teaching game should be full of commentary from beginning to finish, in my opinion.
Also, as a by-the-by, I think the best method of teaching in a teaching game is when your sensei prompts you now and then to look for the bigger move. Weak kyus like me are always not seeing the wood for the trees and playing small moves. Seeing the bigger moves is vital to make progress.