What is the nuance of 很好? Beginning textbooks say 很好 means "very good," but soon add that 很 is usually just there to balance the phrase and does not really mean "very" in this context. S.Rhee's comment cites a much fuller explanation of this grammatical point.

Can 很好 be used, for example, to simply say a young student's behavior is "good enough" without saying "really good"?  For comparison I think of the French phrase "assez bon" which a dictionary will say means "good enough," but in fact it is a criticism meaning "hardly good at all."   

Actually grammarians have a concept of dubitative particle, for a word that makes a superficially affirmative sentence express doubt.  The French "assez" often does this.  I wonder if 很 does this in Chinese, by highlighting that you have not said 非常 or 真的 or other things you could just as easily say.

I do know that when people say 你普通话说得很好 it is a compliment, not an implied critique.  But of course when they say it to me it is merely kind, not true at all, and it never means 真的好.

Question Question about &quot;很好&quot; deals with this phrase but only in the context of a greeting and reply.
I made mention of this on my answer to a different question here (which I'll repeat here for your benefit): https://chinese.stackexchange.com/a/890/166

In general, adjectives in Chinese do not need to have 很 to be used predicatively (and thus 很 is not be considered a copula); however the lack of 很 gives the implication of contrast by default. Chinese: A Comprehensive Grammar by Yip Po-Ching and Don Rimmington explains it in this way:


  The presence of these degree adverbs and complements removes any implication of contrast that is latent in an unmarked predicative adjective.
  
  If somebody says:
  
  
    这本字典好。 zhèi běn zìdiǎn hǎo. This dictionary is good.
  
  
  the speaker must be understood to be implying that some other dictionary is not as good as this one. In fact the degree verb 很 hěn 'very', unless it is emphasised, does not really mean 'very', and its integration into adjectival predicative is more often than not to counteract an implication of contrast.