I've been looking this week at presentation from the Workshop on the Future of Virtual Execution Environments hosted by IBM Research two weeks ago. Most of the presentations are online.
Reading through the slides you can get a good idea of what different people in the industry, academia and the open source community view as important issues in implementing virtual machines. The goals and the means of achieving them vary from one implementer to another. Most of the talks did not have much detail but they are a good starting point.
Dan Sugalski wrote a summary of the workshop and that was a good read too.