Just looked at the blogpost and reading through the documentation, it looks all very exciting.
One thing I would point out as being important for long time developers is the following:
Prior to 4.0, you could use both AMD and legacy modules. Beginning with 4.0, support is only provided for the AMD model.
So everybody check this out http://blogs.esri.com/esri/arcgis/2013/10/14/the-abcs-of-amd/
On thing I find annoying/different right off is that the map only reacts to the mouse wheel on the fourth click/roll of the mouse wheel. Where as in the 3.x API every roll/click of the mouse wheel caused a zoom in/out. Is this configurable somewhere and I am just overlooking it?
Steve Cole - you are correct, in beta 1, the KMLLayer isn't supported. This is the first of several betas, and many classes are not yet available, but will be available in later betas. As mentioned on the beta home page and the blog post - the first beta is primarily a developer preview with limited functionality.