These are exciting news, especially about Draft.js editor integration! I've built multiple projects with Wagtail CMS, and it's always been a great joy to work with the code as a developer. Clients also like it for its sleek and uncluttered admin UI.
Played with Wagtail and the first thing I wanted to find out is how to customize the wysiwyg editor. Is there any API to add / remove buttons, markup, etc? Is it possible or we're stuck with what's offered?
It's not quite possible yet, but that's a frequently requested feature, and we're most of the way there. The wysiwyg component we're using, http://hallojs.org/, supports defining extra buttons via plugins, and Wagtail provides a hook mechanism for passing in custom JS/CSS to the editor - we're just missing a bit of glue to connect the two.
I'm cautious about overloading the rich text editor with too many features - things like tables, say, would be better handled as an explicit part of the page model / template rather than kept in an opaque blob of HTML - but I can appreciate the need for more formatting options.
The same can be applied to Rails (let's compare to it) too:
1) You're at the mercy of the gem developers and Rails changes versions faster than Drupal, breaking backwards compability.
2) Number of gems also does have an influence on Rails app performance, at least on an app's startup time.
Keep it up!