Flexible problem solving: Aside from executing arbitrary code at top-level and serious metaprogramming (both of which I sorely miss, coming from Ruby), Dart is a very flexible language. It embraces type inference, closures, top-level methods instead of one liner abstract classes. You're not going to find any amazing stuff like RSpec, Rake, or the Rails Router coming to Dart, though: the language is simply not that flexible. But if metaprogramming / DSLs / etc aren't your thing, Dart kinda lets you barrel through.
Structured ecosystem: Dart has its own package manager, coding conventions, baked in "dartdoc" comments-as-documentation -- all the things you'd expect of a 2014 language. JS is still waffling: RequireJS or CommonJS or AMD? Bower for the client, npm for the server, and you sure as fuck aren't going to be using a single package from either source for both frontend and backend dev.
Batteries included: they packaged pretty much everything you need to get running quickly.
The library / parts syntax: the way Dart handles including files in a library is very bizarre. The library declaration is straight-forward: `library foo;` in your library file. Simple right? You also have to specify that a file is part of a library, too; that's fine: `part of foo;`.
Off to the races, right? Wrong. Because you have to go back to the library file again, and then add `part 'filename.dart'` as well. For every file in your project.
This is obviously for the compiler's benefit, because any human being wanting to know what file belongs to what project would just look at the source tree on the file system, like every other sane language on the planet.
This is flexibility without convenience. You have flexibility in that you can, if having downed a six pack and are now in a drunken stupor, mix several different libraries and their implementation files in one directory and make out OK.
But you don't have the convenience of declaring all files in a directory to be part of your library, even though that's what 99% of how everyone everywhere lays out their source tree.
It's good to be flexible, but it's bad to be inconvenient.
Mirrors API: Mirrors are analogous to C#'s reflections API and Java probably has something similar. There's not a lot of affordance in the API: using `dart:mirrors` means you are going to be writing a lot of code unless you are using it at its most base, simple level.
No generators: Dart doesn't have generators, so
UPDATE: Dart actually gets around this by using lazy iterables: where() returns a lazy iterable, and map() returns a lazy iterable based on the previous iterable. Thanks to +Lasse in the comments for pointing this out.
"The Editor Will Do It": Dart relies heavily on using an editor of some kind. The Dart Editor is great for starting, but its weaknesses show when you move beyond 1-2 files for a project: there's no built-in keyboard-based navigation, no source control integration, no syntax highlighting support / integration for LESS or SASS or HAML or Slim or any of the popular templating languages -- despite being built on the Eclipse platform.
A plugin exists for Eclipse, but it feels like a red-headed stepchild. I finally figured out how to import the files of an existing Dart project into my workspace by creating a "new" Dart project; when it asked me for the "project name" I just put the directory the existing files were located in, and walla, fooled the IDE I guess.
I have a licensed copy for RubyMine, and thus can use the Dart plugin, but it needs serious love. It's obvious whoever is developing that plugin has never used it for anything other than ensuring the plugin seems to work.
Maybe it's just me, but having to stop coding and mouse around clicking the '+'s and '-'s on a file picker to find the file I want consumes an inordinate amount of time and makes it hard to keep track of what I was doing. CTRL+SHIFT+R and done, son.
In the end, Dart is still a respectable development choice. With ES6 being god knows where, doing god knows what, with god only knows how long until you can actually "use" it, Dart is a strong contender as long as you don't need that JS magic.
But goddamn do I want to use a Light Table supported language.