Gershwin himself considered P&B to be an opera, though it wasn't performed in an opera house till the 80s, I believe. The first Broadway revival cut of a lot of the "operatic" elements, making it more "Broadway."
Pirates of Penzance, an operetta, has been on Broadway.
Several other Broadway shows are considered to be very close to being considered in the opera genre such as Most Happy Fella and Candide.
"If there was a Mount Rushmore for Broadway scores, "West Side Story" would be front and center. It snaps, it crackles it pops! It surges with a roar, its energy and sheer life undiminished by the years" - NYPost reviewer Elisabeth Vincentelli