If we talk technologically & not app availability, there's all kinds of things one might do with a better desktop. Academics & random people online stay dreaming up better UI's or other capabilities. I'd like the internal control + self-modification of LISP machines, concurrency support of BeOS, capability-security of KeyKOS or CapDesk, maybe persistence of app data like KeyKOS, reincarnating drivers in user-mode like Minix3, modifiable-for-workload schedulers that prevent one task from taking down system like RTOS's, versioned filesystem like OpenVMS, two motherboards like SGI with clustering like VMS for mission-critical desktops, mini-version of NonStop for desktops for more mission critical, Amiga-style hardware offloading for key stuff like I/O, Burroughs-style CPU that made most code injection or type errors impossible, jumpers write-protecting the chip for my open-source firmware, ECC RAM, and a RAM disk w/ flash backup for main OS & apps to load crazy fast (or it always hibernates w/ hardware acceleration of that or persistance of changes).

That's just a few off the top of my head. If it's a netbook for browsing, it might also use something like Illinois Browser Operating System (IBOS) as its base. Definitely throw in a NUMA chip on the high-performance version, too, so I can finally have me a modern SGI Onyx2 or Altix with 256+ CPU's, TB's of RAM, several GPU's, and a bunch of FPGA's. All hotswappable so my games, simulations, or recompiles of kernels aren't interrupted by mere hardware failures.