As I said, I do not know enough about rap/hip hop to tell what makes it compelling and what not, but I do not find most of the stuff that I hear in the US not very compelling at all. I also understand that rap catching up outside the US is a very recent phenomenon - something that probably has more to do with advertising and promotion than with its actual appeal. Last time I was in Kenya and S. Africa (ca. 2001) I did not see much evidence of hip hop popularity there, but again things may have changed.
As far as the aggressiveness of the US culture and population is concerned - I think it is a particular form of aggressiveness. While there is certainly a lot of actual aggression and violence all around the world - the US aggression is mostly STAGED rather than actual. That is to say, US-ers seem pretty docile in face-to-face interaction and everyday life, but then engage in conspicuous displays of aggression to show off how tough and assertive they are.
I first observed that phenomenon when I worked as an instructor for the US Army. I saw a lots of guys showing off how tough and aggressive they are, to the point that at first I thought that they are some sort of hyper-violent psychos, until I saw them under situations of stress, when many of them actually cried like babies. I then realized that the conspicuous display of aggression and violence is simply a show to mask weakness. I then had a chance to see that phenomenon in other aspect of the US life - such as internet "debating" where you can tell whether the writer is a US-ers by the meanness and asinine nature of the postings, SUV buying, and yes, hip hop and punk music.
I am personally turned off by violence - I avoid thrillers and action movies, and do the same with punk and rap. But I find these displays of cheap macho pseudo-aggression rather pathetic - a sign of weakness and cowardice. People with real power and strength do not need to test their prestige in the pizza line, so to speak, and show off how aggressive and tough they are.
I think that there is an inverse relationship between the level of the perceived personal inadequacy and weakness and the staged aggression and meanness to compensate for that weakness and inadequacy. I think that feelings of weakness and inadequacy come mainly from alienation, and the US is a very alienated and alienating society, hence its infatuation with displays of aggression and violence. But it does not mean it is not present in other societies - just that the US seems to have more of it.
Wojtek