Crystal is great! It's still being under development but there's not many breaking changes after 0.20.0+ and it's getting smoother day by day.
Parallelism and Windows support are on the way. BTW feel free to join the superb friendly community on Gitter
https://gitter.im/crystal-lang/crystal :)
[Shameless plug] I'm the author of Kemal(http://kemalcr.com/): Fast, Effective, Simple web framework.[/Shameless plug]