I do know that for pop music in the US and UK, often formulas and techniques are followed to "guarantee" a hit. A potentially good original song is tweaked and restructured to appeal to a wider audience.
There has been some fascinating psychological research on what makes a pop song successful, however reading it is a bit depressing. You realise how much creativity has been sacrificed for the sake of a few extra dollars. Boo.
If you have the time, find and read "The Manual" by the KLF. It's part-satire, but it is a guide to having a number one hit in the UK... written in the 80's, after this musical act had deliberately done so. Their method has nothing to do with creativity (well, it says "number one hit", not "a great song").
I'm curious to know if any similar research or theory has been done in other countries?
And then there's always the pop promotion machine, making people like an otherwise average song... ;)