One thing that both annoys and amazes me is the limited number of chord combinations in most songs. It's extremely frequent to hear a musical composition and get that feeling that you heard it before. And we are probably right most of the times.
In the last AC/DC album there is a song entitled "Big Jack". As soon as i heard it i immediately recalled another song by a very polemic Portuguese singer from the the 80s: AntÃ³nio VariaÃ§Ãµes (look up his songs, they were pretty innovative back then, actually).
I still trying to think why i am writing this but guess that i can't help wondering, first, why aren't there any more musical notes (i guess i should look that up) and, second, why most bands don't explore more and keep falling to the same chord combinations all the time. The easy explanation is the amount of money that you can make with little work on a catchy pop song, but i feel sad if that's the only one.