I am a 20+ year senior architect with a passion for anything that brings productivity to the area of application development and Dart ranks extremely high on the scale of both client and server side web application development. I have 14 years of .NET development experience and came to love C# and all the large eco-system that was developed to support implementation of any web application came to need. Thing was that when I adopted Silverlight I was spoiled with the richness of RIA based applications but as time rolled on it became very clear that JavaScript won the web war and so I researched many years to conclude that Dart has most of what I had and is a great place to camp out for a while, contribute to the every growing developer community and such.
As for what it lacks, I'd agree with most other comments so far however I also feel that all concerns will be addressed especially the JS INTEROP concerns.
One of my main concerns is that of resultant script size. I think (but I am not 100% sure) that the advent of lazy-loading support for Dart should mitigate some of these concerns along with continued updates from the Dart team.
Otherwise, if you have not tried Dart, and you are a skilled Java or .NET developer, then you may want to test drive it as you will pick it up in an evening, and if your anything like me, you will never want to leave it!