Thank you very much for your thoughtful feedback. We're pleased with the UI but obviously it's not perfect - I've just started using slack.com which is also pretty flat but feels like a new benchmark for usability. I'll discuss it with the design team, and if you have any more specific feedback we'd love to hear it.

There are still a couple of Wagtail features which rely on contrib.admin. We have to decide whether to smarten up the contrib.admin UI or pull them into the Wagtail admin. Is your CMS available somewhere public?