Awesome Music: Not only does every song on the soundtrack sound like a likely hit song from its respective era in which it was supposedly written, you're very likely to keep humming various motifs for days after you've watched the movie.
Better on DVD: There's little argument that American Cox: The Unbearably Long, Self-Indulgent Director's Cut is the superior version of the film, giving most of the jokes more breathing room and setup. That said, the filmmakers argue that its biggest drawback is how it makes Dewey out to be more of a jerkass than a well-meaning idiot than they would have liked.
Memetic Mutation: The movie parodies the tropes and cliches of music biopics so effectively that it's basically become impossible to make one nowadays without someone, somewhere, pointing out that Walk Hard has already made fun of it, with a handy quote at the ready to prove it.
Special Effects Failure: When Pa's upper half keels over, you can see the rim of the hole in the floor where the rest of his body is. Given the nature of the movie, plus how incredibly fake his lower half looks, this was almost certainly intentional.
Tear Jerker: For such a goofy movie, it has some rather tender moments.
Dewey sings his masterpiece... then dies three minutes later. Funny as it is, the Comically Serious framing still makes it feel like we've watched the life and times of a talented, prolific artist go out on top.