If you ask me, it's the combination of browsers and websites not properly exposing the existence of RSS that has stunted its growth.
Firefox used to have a button for subscribing to any RSS feeds it found in a webpage in the navigation bar. It was the only browser that did it (EDIT: that I was aware of - others have pointed out Safari also had this feature for a while), and it was a fairy obscure button too. Then they removed that.
So now you actually have to hunt for the RSS feeds - and since many webpages use a gif saying "RSS" (meaning text search is useless) that isn't always easy.
It's not difficult to use at all, but it is certainly a very obscure feature of the internet. That halts adoption.