* Dart compiles to JavaScript, Silverlight did not.
* Dart is actively being developed, Silverlight is not.
* Dart does not require any plugins, Silverlight did.
* Dart natively supports the DOM, Silverlight did not.
Except:
1. The Dart VM isn't included in Google's generally-released browsers, either (it's included in Dartium, the special version of Chromium bundled with the Dart SDK).
2. The Dart VM isn't a plug-in.
3. As Dart compiles to JS, you don't need the Dart VM to run code developed with Dart, you just need a browser that supports JS.