What HKT would let you do is to write a generic "try" function that works for all "error-like things", instead of having to have a separate try! macro for every error type. Except that that isn't the case any longer, because we now have the FromError [1] trait, which means that in practice try! works for custom error types as well.

Instead, what HKTs would actually be useful for is something else entirely: generic algorithms that need to (for example) work with different kinds of collections' iterators and can be specialized for them at runtime, without paying the price of allocating the iterators on the heap. For example, consider a graph algorithm that's parameterized over the type of graph. (Note that I've never actually needed to write one of these algorithms in a generic way myself, but I can see it being useful in some circumstances.)

What HKT is not there for is Haskell-like monadic "do" notation. That simply isn't idiomatic Rust, and I don't foresee it becoming so in the future. The try! macro is the way you do this kind of thing, and it's already shipping today without HKT as a type system feature.

[1]: http://doc.rust-lang.org/std/error/