Are the performance benefits negated by the "compiling to javascript" part?
The greatest problem with ECMA Harmony is one that has plagued (and will continue to plague) the computer world for generations, and that is Backwards Compatibility. I see dart as way to do away with the old.
That said, the greatest challenge would be to ensure it gets picked up fast, and it won't be done through developer adoption, but through cooperation with other vendors, and I don't see mozilla and microsoft rushing to adopt Dart, even if it's blazing fast and has a writeMyProgramForMe() function.
The review linked above does point out some very interesting points, especially about trying to fork the web community being unhealthy, though I agree with google's assessment that javascript has fundamental flaws that are unlikely to ever be fixed (especially if the ECMA team decides to keep backward compatibility, as seems to be the current mindset).
I mean, learning dart now would put one in a VERY advantageous position if it were ever to gain momentum, but if no one but chrome adopts it, it will remain forever a javascript translator, akin to coffeescript, which would just be sad, so I'd appreciate if someone could point me to a discussion on those topics (as I'm sure someone thought about this already).