Big fan of alternative OSes. I would add the word: viable to alternative/open platforms.

In lots of use cases this just means a modern, up-to-date, browser. Firefox OS had the potential to become instantly viable but Mozilla (I feel) was working with blinders on.

Haiku and ReactOS are at the cusp. ArcaOS (the latest OS/2 incarnation) would be jaw-dropping exciting if somehow (not likely) they went Open Source.

Aside from the major commercial players, distroLinux and the BSDs... I don't see lots of viability out there (sadly). Maybe Android x86 (though it's frustrating to use as a Desktop OS)...?