So many interesting/fascinating concepts and ideas that will remain relegated to niche communities because of basically one problem: lack of drivers. This is the problem that must be solved if we ever want to move away from the dominance of a few OSes "too big to fail" and their million compromises to a thriving field where competent programmers can create new OSes as easily as now they can create new apps.
By the time you have attracted enough users that those few percent of performance you are losing on the paravirtual drivers is an issue, the extra code for some performance critical native drivers will be a small part of your OS.