I think, to me, the difference between SOMETHING ROTTEN! and the plethora of other, better meta-musical comedies is that "meta" jokes only work when they are used to say something. Like Kad's cabinet full of marshmallow fluff; you need to have meat on the bones. So, THE BOOK OF MORMON soars high on its reference jokes (and its shocking-to-some profanity) but it's in service of a really interesting idea about the fundamental necessity of and inspiration for religion. THE DROWSY CHAPERONE has a marvelous framing device that provides a structure for the meta-jokes to bounce around in, but not overwhelm and subsume the entire show. [TITLE OF SHOW] found freedom by dropping any pretense and making the metatext the text. ROCK OF AGES just came here to party, and knows you did too, but the meta-jokes work because the score itself is meta-, based as it is in a popular era the audience knows and (hopefully, for them) loves. Ditto for SPAMALOT, which is expanding to Broadway scale a beloved little cult film... the meta-jokes work there because it's Monty Python saluting the legacy of the new form they're barging in on. (That's also key to MORMON's success, as well.)
SOMETHING ROTTEN doesn't have a there there. Jokes aren't just referencing and expecting a laugh; it's what you say about them that's a joke. Take "A Musical," for example: the LES MIS gag is a true joke. It's a groaner, but it's actually got construction and punchlines. Too many other references, however, are just references. Putting three sailor's hats on three dancers is a reference, not a joke. Ditto quoting SWEET CHARITY or any other famous musical riff. Too many jokes, songs, and entire characters exist solely because you can hang jokes off of them. The jokes have to serve the story, not the other way around. The jokes in SR! overwhelm the material, and in doing so, point out how little beyond the jokes there is.
Words don't deserve that kind of malarkey. They're innocent, neutral, precise, standing for this, describing that, meaning the other, so if you look after them you can build bridges across incomprehension and chaos. But when they get their corners knocked off, they're no good anymore…I don't think writers are sacred, but words are. They deserve respect. If you get the right ones in the right order, you can nudge the world a little.