Wicked Mimi, while I do think you are missing something, let me offer my explanation of why one of my absolutley favorite shows may not appeal marginally to some.
1) Mel Brooks. While he is a comic genius, the type of humor the show consists of is mostly offensive, crude, or satirical. If you cannot handle those very specific genres of jokes, then there's little chance you may be able to relish in the delights of the show.
2) The show is purposely old-fashioned. While working in the confines of June 1959, the show evokes a very classic traditional musical comedy feel, like that of Guys & Dolls. It is no wonder, then, that the score is a limiting factor in terms of audience.
3) There's no spectacle gimmick, like that of a chandelier or flying harness. Other than the fantasy sequence, the monstrous sets never step out of theatres or offices in New York.
It's always best to know what you're getting yourself into. I really can't relate to people who blindly buy tickets without at least a smidge of background on the show. Some may decide at that point that there's no need to proceed. Of course, it's all a matter of personal taste. Though, I can't imagine not buying into at least a touch of the show's outlandish fun.
...And so, there I was at the stage door for "Equus". The enthused avalanche of "Harry Potter" fans was literally pushing me into the barricades. As I was thrust face-first into Daniel Radcliffe's hat, I suddenly felt the thumb of a lanky gentleman behind me pressing firmly into my back. It was then that I realized that both of his hands had Playbills, and that both arms were outstretched in front of me...