I know syntax is subjective, but some of the choices seem a bit odd. For example, declaring variants and using their constructors looks like Haskell, but the semantics is still OCaml. In Haskell, constructors are first order so they can be passed as functions, and partially applied. It makes sense that their declaration and use looks like function declaration and function calls. In OCaml they are not first class, that is, you can't pass the as arguments, or partially apply them. That's why it makes sense for the declaration to look like a tuple, and the use to look like a function applied to a tuple--well, somewhat, you can still argue that it's still confusing because you might expect to be able to apply the constructor to a tuple variable, but well, such is life :). Unless constructors are first class in Reason--it doesn't look like it from a quick scan through the docs--this particular syntactic difference is of dubious value, and, worse, it can be misleading to newcomers.
Also, changing `match` to `switch` seems gratuitous as well, and it also loses some of the meaning of the original. i.e. "I want to match this value against this set of patterns".
Finally, I know that using `begin` and `end` for blocks is verbose and Pascal-ish--which people seem to hate for some reason--but using { } for scopes looks out of place, and leads to awkward cases like this:
try { ... } { | Exn => ... };