I am a native English speaker and have gone through 8 years of grammar school (and 20-some years of speaking...). Nevertheless, sentences such as the one in the title have always sounded odd to me and I've never been sure if they are in fact grammatical.
To explain my question more generally (and perhaps helpfully for ESL students), in English, nouns that represent generic quantities (most, some, all, etc.) can be considered either singular or plural depending on the context. When the context is not clear, whether those nouns are singular of plural is often determined by a prepositional phrase.
Most of the cows are white (most is plural)
Most of the cow is white (most if singular)
This is relatively clear and straightforward. An interesting problem occurs though when a singular "most" is composed of something plural (as in the title of this post).
In that case, the "conversation" (a singular noun) is composed of "complaints" (a plural noun). So to convey the correct intention, it seems necessary to construct a sentence that has a singular noun in the subject, and a plural noun in the complement which goes against subject complement agreement rules.
Furthermore, the possible alternatives all convey something different:
All of their conversations were complaints (wrong because it states that there were multiple conversations)
All of their conversation was a complaint (wrong because it only states that there was one complaint and I wish to state there were multiple)
Is this in fact an exception to subject complement agreement rules or are sentences like the above unacceptable in proper English?