The first issue with the film is that there really isn't a likable person to be found. The best we can do is Grace Kelly's character, who is having an affair with a bland, vanilla writer from New York named Mark. Hooray for infidelity! How does she justify their continued tryst? Because "Tony has changed." Well, yeah he has; but so has she, in that she's sort of sleeping around. (As an aside, the more general idea that the lovely Grace Kelly would need to slum it with either of these clowns is what actually stretches credulity to the breaking point. But I digress.) And obviously Tony's no saint either, since behind his debonair front he's plotting to have his wife murdered. And this isn't a rash, heat-of-the-moment thing either; he's been planning it for quite some time.
But Hitchcock is often playing around with infidelity and unlikable characters. No, a larger issue is that there just really isn't much action here, and even less mystery. The biggest moment of suspense comes from trying to hide a door key on the stairwell. We spend most of the movie just waiting for the bad guy to get caught, and all of the various drawn-out conversations simply pad the time leading up to that inevitable outcome. That may not sound like it could ever be terribly exciting, but in Rope it actually was. So what makes this one different? On top of the fact that we don't really care about any of these people, the murder and its cleanup occur quite early on, and then we're just stuck with verbal sparring and theorizing. At least in Rope there was a dead body right there in the room whose discovery could at any moment blow the whole thing. Tension! But here? Eh, some tension... I guess. It's more about if someone's lying can outlast a couple of "it just doesn't feel right" vague theories.