Flaws?
I think this movie is fantastic, and deserves a serious re-evaluation. However, some scenes keep it from being top-notch Altman.
For me, the big failure is the movie's climactic scene immediately after the car crash. When the groom's father and bride's mother start arguing about money, and wedding guests, etc -- even as they think that their children have just died. The scene rings so false and flat, and you can really feel, for one of the few times in the film, the actors trying so hard to improvise.
Then, when Dino and Muffy arrive and everyone is relieved, they express no surprise that their two friends have died.
Just wanted to get that off my chest! (I've just watched the movie on the Movies! cable channel.)