If you're already familiar with coding
Java, then you'll find coding Dart a breeze due to it's class based nature. You can write Dart code that looks almost like Java. Example:
You get the advantages of JavaScript (ie, browser-based) + jquery (consistent access to the DOM with the dart:html library), node.js (Dart has a server-side VM as well), and sane library and packages.
With regards to Ruby, Dart has some of the similar functionality, especially around "noSuchMethod" - which is similar to Ruby's method_missing (which helps with much of the magic behind Rails).
So, Java-like productivity in the language, javascript+jquery access to the browser, nodejs like Dart VM on the server side. What's not to love