HScheme is intended to be R5RS-compliant when run with the default monad and bindings. As of 2007-06-26 22:28, these issues stand before release:
Macros are not hygienic.
Ellipses don't work in vectors in syntax-rules.
The quasiquote implementation isn't standard.
The procedure eval exists, but no environments are available to use it with.
I expect there are missing and broken procedures and macros. I haven't yet checked every one.
Also, the parser is very slow, many times slower than it needs to be.