> In practicality how difficult/easy is it to build cross-runtime libraries and how much reuse can you get out of clojure libs b/c of its hosted status?

It depends on what you're trying to do, and whether the platforms help.

Doing something 'pure' like math or datastructures: trivial. Writing a common interface to two different host libraries (e.g. DateTime in JVM & Browser): manageable. Doing something 'host-y', like graphics, with no host libraries in common: very hard.

Basically, Clojure doesn't get in the way of cross-platform concerns, but it might still be a difficult problem.