That looks interesting. I've got a CMS which shares some philosophical similarities to Wagtail. It's a lot clunkier and uglier though. I wish I had the time to add some polish. I think the 'pages have types' idea is spot on and something all the other Django CMS's I've seen seem to get wrong. (too much freedom for the user to create 'Frankenstein pages' by arbitrarily combining blocks).

The editor and image library is very nice.

I feel you've gone slightly too far in the 'flat and beautiful' direction. Some UI elements lack affordance and I think usability has suffered slightly at the hand of aesthetics.

Finally I still feel there's a case for not completely replacing contrib.admin but that is a complex debate and I can see in your specific case you probably made the right decision. I'd really like to see if I could achieve some of your elegance without abandoning (or reinventing!) contrib.admin.

Best of luck!