A few things to respond to in your post, Toon:
The audience reaction (going wild, as has been posted) could be to the material more than it is to the actors. People love the show, even if it's not the best cast to ever grace that stage.
When those actors are bored, their performances often fluctuate. You could see them one night and they could be dead on, and I could see them the next night and they could be like couch potatoes up there.
Yes, I think it's valid to say that someone who hasn't seen another, perhaps "better" cast would not think to criticize so harshly, but I also think that if an actor is truly bored, going through the motions and phoning in his performance, noticing that doesn't exactly require rocket science. I will say, though, that is is all very objective, so I'm not *totally* disagreeing with you, but I think it's more to the (dis)credit of the actors than your post seems to indicate.
A work of art is an invitation to love.