The first one that comes to mind is, as stated before, 9 to 5. I bought tickets for the Broadway run after reading the excellent reviews on this board when the show was in LA. Then it came to Broadway, and even most of the posts on here were positive then. The show opened, was reviewed by critics, then died, and people sort of stopped championing it...I still liked it, and didn't regret buying tickets, but it was certainly overestimated.
Other things I can think of are Cry-Baby, A Catered Affair, etc.
Also not exactly overestimated by posters here, but I remember the first time I heard about Glory Days was in an article (on braodway.com, I believe) entitled something like "New Musical Glory Days announced- could it be a surprise Best Musical Winner?" or something like that. That is the biggest overestimization ever, I would think...
"Art, in itself, is an attempt to bring order out of chaos."-Stephen Sondheim