I'm no language expert, but the there things I can think of that make bel impractical without major compiler trickery are (1) lack of primitive hash tables (2) lack of primitive arrays (3) no support for tail call optimization (though that third thing is probably fixable with the right compiler tricks)

The other concern I have is the lack of a literal associative data structure syntax (like curly braces in clojure) It seems that would negatively impact pg's goal of "code simplicity" quite a bit.