it's hard to think of too many shows that are boring in and of themselves. the only times I've been genuinely bored at the theatre, it was more a result of the performers or the direction. not to say that every show is good. there's plenty of bad theatre created everyday. I just think every show has the potential to at least be interesting if worked on by the right people. Rent is a good example. Back when there were still good casts in the U.S. it was a truly exciting, inspiring night at the theatre. Now that the show has lost its freshness and the casts are uniformly sub-par, it can be a really boring show to sit through.
"Secret Garden" comes to mind too. It's one of my favorite musicals, but it can be very boring if it's not done extremely well. An excellent production of TSG is hard to beat. Anything less than amazing, even a "pretty good" TSG, is booooorring. The same is true of Fiddler, at least IMHO.
"The last train out of any station will not be full of nice guys."
- Dr. Hunter S. Thompson
"I wash my face, then drink beer, then I weep.
Say a prayer and induce insincere self-abuse,
till I'm fast asleep"- In Trousers