Nice to see I'm not the only one pondering this. The above answers do sum up the most frequent usages, but I would like to propose an addendum concerning the ambiguity of the phrases, which, for me, was the core of the question:

"Anything but" may sometimes also be used in a way that is closer in meaning to "nothing but", mainly when posed as a rhetorical question, as in:

"How can B mean anything but A?" - implying that B means A, and only A

This does not call for avoidance of the phrase, however, as the intended meaning can, more or less, be easily understood from the context of its use. Give readers and/or listeners some linguistic credit... ;)

Nice to see I'm not the only one pondering this. The above answers do sum up the most frequent usages, but I would like to propose an addendum concerning the ambiguity of the phrases, which, for me, was the core of the question:

"Anything but" may sometimes also be used in a way that is closer in meaning to "nothing but", mainly when posed as a rhetorical question, as in:

"How can B mean anything but A?" — implying that B means A, and only A.

This does not call for avoidance of the phrase, however, as the intended meaning can, more or less, be easily understood from the context of its use. Give readers/listeners some linguistic credit.