"Compatibility" has killed OS research, because the OS's main job is to multiplex the underlying hardware among applications, and it seems that the current OS's do that well enough.
I think that "systems research" has today moved towards building infrastructure on top of VMs (Java / .Net). There's a lot of innovation there which would have before went into a new shiny (and incompatible) OS. (Example: Java security policies.)
Also: writing a new OS requires a lot of upfront investment with _zero_ research value: boot/startup code, managing the CPU's idiosyncrasies, _drivers_ for bazillion of different HW units, a filesystem, TCP/IP stack, etc. No wonder people just tweak existing systems, as it's almost impossible to rip out these boring parts from Linux/*BSD kernels and reuse them in something new.
My personal wish for furthering system's research of today is something that would kill the need for hypervisors. Android, for example, runs each app under a different UID, thus achieving isolation w/o hypervisor. Let's devise something similar for desktops/servers; bonus points if the system can run existing binaries.