Multicasting is an engineering issue, and a very complex one. There are tricky engineering routing problems. Major ISP's are having enough trouble with routing as it is without introducing this kind of extra complexity, so it won't make it to the top of their list for a while. Until it does we won't see multicasting broadly deployed on the net. Check out the draft RFC's prepared by the Internet Engineering Task Force's working group on Large Scale Multicast Applications to get some sense of how multicasting interacts with simulation applications, for example.
Quality of service is much more problematic.
The service provider must reserve chunks of network resources to be sure you get enough for your application. There is a patchwork of internetwork service providers. We may be able to get a quality of service guarantee within their network, but it's very tricky to do it between networks. This gets deeply into billing and capacity planning issues. The networking people, talk about quality of service at the network services level.