How to describe the "not bad" expression?

How can you describe the "not bad" expression?

edit for link problems

http://alan713ch.tumblr.com/post/110765650002/thechronicleofshe-corriezodori (warning: shirtless men and women in bikinis)

edited 12th Feb '15 4:27:09 AM by Bluelantern2814

I usually describe it as "a mildly impressed expression" or depending on the case, an "ambivalent expression".

Depending on the character, apathetic could work. It depends on the context of the 'not bad'. The difference between "Not bad. That was some fine work!" and "Not bad, but I think you can do better".

The context (edited for politer terms and clearer situation)

Brothers A and B bring guy C home, A has only been with women so far. Their father is there and B says sunnily

"A wants to sleep with C! Also, hello"

Father reaction is the one displayed, the internal thought process could be described as surpresed and "I have to agree that is a fine choice as far dudes go"

It is a weird family.

Are you looking for the verb of what it was, like "C grinned and said-" or "-said C, grimacing."

while I'm not sure if sturgeonfacing is what the expression is called it seems to fit the Not Bad you were asking about.

edited 15th Feb '15 1:24:23 AM by Aespai