I think another indication of the depth and layers in a movie scene is the amount of debate generated. A lot of times this comes about because the filmmakers didn't spell out every single plot point and they left something of a mystery. The film viewers are then left with their opinions (which they can then have fun debating about with others on message boards like this ).

For instance, if Fredo had lived, would he have betrayed Michael again? Who did Michael really love, Apollonia or Kay? Did Roth intend to kill Pentangeli, or did he stage the attack to fail? Did Johnny Fontane hesitate when Michael asked him to sign a contract to appear in Las Vegas? Was Geary defending Michael or accusing him in his speech at the Senate hearings? All good subjects of debate because they are matters of opinion (and even that's debatable ; many think there are definitive answers for some of these questions).

It must be a difficult balancing act to be a filmmaker. On the one hand, if you feed the audience too much information, you detract from the air of mystery; if all questions are answered, then the movie is less likely to resonate in the viewers' memories. On the other hand, if you don't give the audience enough information, the plot becomes unclear and you lose the audience (echoes of GF3). GF2 hits just the right balance.