Well it ain't Flex Viewer.
In this Builder there is no Table of Contents. We are restricted to 3 extra widgets. And the customization in the first beta with JSON where we could configure position and some other quick things... appears to be gone? Also where do we add our own REST layers? I saw that mentioned in What's New but can't find it. Is there an "Advanced Mode" I'm missing somewhere or is this really all there is. Even BasicViewer template was more useful as a starting point. That is just the problem. ESRI by now has released dozens of "starter apps". HTML5 boilerplate. Bootstrap starter. Bootstrapmap. AngularJS viewer. SimpleViewer. BasicViewer. Now this Builder. The problem? All of them are lacking some fundamentals. Table of Contents etc. Easy use of our own layers. Yes...... I know I can dive in and remove the hardwired restriction to 3 widgets somewhere I'm sure, or add my own layers. But then, what is the point of this template!? If it's only for extreme beginner users, they're better off just sticking to ArcGIS.com. This is in a useless middle zone, between beginning and advanced. It's basically a dumbed-down Basic Viewer with a graphic user interface to set it up.
I hope they add a lot more to this before release. The easier JSAPI is, the more server licenses they may sell...
The comment on people developing 3rd party widgets to plug into this sounds great though. That's key. Like in Flash. You download it, alter one line of code and presto, it works. No scope issues, AMD vs legacy issues, and so on. Like fitting two legos together. I hope ESRI makes Widgets finally this easy, with this builder. First one to publish ags TOC widget will be pretty popular here on the forum!