It is difficult to figure out what's localized or application specific , and what's global and truely general purpose. E.g. DNS is a form of middleware which is truly global. It's less clear to me how global they will be and how general their role will become. Some things are clearly part of middleware, however. We need public key infrastructure for global authentication.
Authentication is a major barrier right now.
There is a lot of foolishness like IP filtering which doesn't work well because people access resources from home and office. They discussed negotiation services where you have an application that has to do quality of service to multiple sites and needs synchronized services, for example, as a possible example of a new middleware service that the next generation of applications will need. It is important not to get bogged down in a general theory of middleware. Thinking hasn't yet matured to the point where this is likely to be a very concrete discussion.