Why do some blu ray's have good PQ and others look bad?

[Movie shy]

Why do some blu ray's have great PQ and other blu ray's look bad? I have a Sony BDP-3200 blu ray player and a 720p HDTV. I know most of you will say it's because of my 720p HDTV but many of my blu ray's look great. For example I bought Cleopatra blu ray and the 4 movie musical collection both on blu ray. Cleopatra looks amazing on blu but The Band Wagon from the 4 movie musicals collection just doesn't look that good. I know bit rates can affect the PQ but the review on this site for The Band Wagon said the PQ was very good but not for me. I see a lot of grain in the movie and the picture doesn't look vibrant. Just wondered what people think?