Two things that come to my mind: 1) General concurrency/parallelism ease of use. It would be nice to have Async/Await a la C#, as well as some easier to use channels and synchronization primitives/functions along the lines of Go or Core.Async. And 2) use of <> for type annotations instead of [], which is much easier to parse when nested.
Servo is definitely demanding a wide variety of approaches to both concurrency and parallelism, so I expect the facilities for such in Rust to mature very quickly (but faster with more help and feedback!). It's a very important issue that is seeing work from full-time developers as we speak.
As for the second point, we've gone beyond the bikeshed (<> and [] are both exactly as ambiguous to the grammar, so it's just an argument of preference by this point) and addressed the root issue with `where` clauses, which prevent you from needing to nest <> at all for a majority (or perhaps plurality, I haven't measured) of function signatures where you previously needed to.
