After exploring the beta version a bit, it seems to have a lot of potential. Here are a couple initial thoughts\questions:
1) Wondering if the concept of widgets will be part of this as it progresses. For instance in Web AppBuilder builder environment, there are a host of widgets, like legends, layer lists, basemap pickers, draw tool, etc... Not seeing anything like that in the beta. The list of widgets seems to be more generic building blocks (row, column, panel, list), which is cool, but I wouldn't have a clue how to re-create a sophisticated widget, similar to what comes OOB with WAB.
2) I really like the flexibility to position interface elements and to create layouts for desktop and mobile. I do think this asks a lot more of end users, compared to WAB. With that, you could spin up a new app with a nice look and feel and lots of functionality in a matter of minutes. My impression of experience builder is that it seems more complicated and more of a learning curve to become productive with this tool.