I don't know that I agree.
If John makes a point of saying hello, he is friendly.
If John doesn't make a point of saying hello, he is not friendly.
If you say hello to John and he does not say hello back, he is unfriendly.
To clarify Barb_D's point, saying John is not friendly means that he isn't friendly. It is not a trait he possesses. Whereas saying John is unfriendly means that he is the opposite of friendly.
I don't know that I agree.
If John makes a point of saying hello, he is friendly.
If John doesn't make a point of saying hello, he is not friendly.
If you say hello to John and he does not say hello back, he is unfriendly.
"The meaning is the same, and they could be used synonymously. #1 would more often refer to a specific instance, a passing mood, while #2 would more often refer to a personality trait." .
I agree with Barb. If a person behaves in a way that is the opposite of friendliness, for example by being rude, they are "unfriendly."
A person can be "not friendly" and also "not unfriendly" at the same time.
As always, context matters. You could use not friendly to mean unfriendly, but decontextualised, I would agree that the absence of friendliness doesn't automatically imply unfriendliness.
FYI, here are a few sources to support my "no difference" in response to the original thread , at face value and lacking additional context: englishforum.com, dictionary.com, and as the primary definition of "unfriendly" (i.e. "not friendly") from Webster's Ninth New Collegiate Dictionary and thefreedictionary.com. And finally (at least from here), consider the following dialogue:
(She) Remember, we're going to dinner at my sister's on Saturday.
(He) Oh no. You know I don't like to go there.
(She) Why?
(He) Because her husband, John, is unfriendly/not friendly. (Either one works for me .)