It's not about what you can or cannot build. It's about how you build it. For example, if you build a distrubuted system on unix or win32, the MPI API is almost the only way to go for you, which turns your code and entire application into complete mess. Whereas, Plan9 introduces a very different approach, which makes your code neat, lean and robust and you can dynamically add/remove nodes without rewriting and/or restarting your app.

I believe there will come some Linus Torvalds into Plan9 world who will revive the system and its ideas. From my point, Plan9 is ideal infrastructural solution for heavily loaded distributed web systems (AKA cloud networking) and other Web 2.0 stuff.

PS. pls don't mention Mosix.