And i don't mean his classical albums specifically, but his classical abilities in general, which can be discerned even in his rock music. As i'm collecting his discography, i like to read reviews and comments on his various albums, and invariably, it seems most of the people who rate his works seem to know very little about classical music, particularly 20th century classical music, or don't particular value his talents in that field. In particular, not a lot people seem to know that much of his music needs to be listened to "vertically", precisely like you would with real classical music, and that a lot of the "weirdness" of his compositions is nothing other than an harmony or counterpoint of colors and sounds, which requires a good talent for orchestration on top of a solid knowledge of the rules of harmony to pull off correctly, and cannot be listened to "horizontally" like you would with a rock or jazz composition. Particularly good is also his ability for thematic development, particularly as melodies are broken up and spread out through out the harmony, just the way a real classical composer would do. But what i find interesting is that he does it in a way that makes everything sound spontaneous or even improvised. Now many classical composers had a similar talent (Mozart above all), but not many 20th century musicians did (Stravinsky being one of the big exceptions, vis his late serial works), and that is what makes his avant-gard so easily digestible.
In a sense, i wonder whether Zappa himself didn't really understand where his real strengths lied (or perhaps he was a bit self conscious about it? Could it be that he really didn't think he could have made it as a real classical composer?), until very late. As a rock musician, he is good but not necessarily the best. As a satirist or social commentator he was so so. He was clever but his enfant terrible antics got real old after a while. Judging from his solos he would have been a great jazz musician but he also approached jazz music more as a Gil Evans of sort than as a soloist. But it is his classical leanings that seem to fill up the "gaps" one finds in his rock or jazz music. Waka-Jawaka is a good jazz fusion album but if you pay attention to the way all the various instruments interact with each other and the spontaneous counterpoint that results from their interplay the music becomes a lot more than just another jazz fusion album. And the same applies to much of his rock music as well.
I haven't looked into this, but i wonder how well he is rated by actual classical musicians. The more i listen to Yellow Shark, the more i think he is in the same ballpark as any of the major late 20th century composers, like Ligeti, Penderecki or Alfred Schnittke, you know, composers who's music is actually fun to listen to on top of being very well put together, unlike, say, the music of a certain Boulez.