One feature which seems to be unique to Fable is that the repl doesn't rely on connecting to a compiler on the server. If I type some F# code, apparently it can be compiled/transpiled right in the browser!
One quibble with the mario demo. The game feel is off; in the game he falls faster than he ascends (different gUp and gDown gravity).
It would be great to read thoughts from someone familiar enough to compare/contrast Fable with Elm.