First off, I don't think Automattic has handled communicating it very well. Gutenberg is going to be a big, seismic change in what WordPress is. It seems reasonable to expect lots of themes and plugins will break when it goes live, especially older ones that haven't been as actively maintained.
And yet, if you surveyed the global WordPress userbase, I'd be amazed if you found even 10% of them were aware that such a change is even on the horizon. The communication to developers around Gutenberg has been hit or miss, but at least they've been making an effort on that front. In terms of communicating to the general WP-using public, other than a notice in the dashboard in the most recent version, I'm not aware of there having actually been any. So there are going to be a lot of "WTF" moments when people update to 5.0 and things start breaking or behaving contrary to expectations.
Secondly, I fear that Gutenberg is going to split the WordPress developer community. WP has always been a PHP project; if you wanted to contribute, whether to WP core itself or by writing themes or plugins, PHP was the language you needed to know to do that. Gutenberg, however, is all React-based, which means now there will be two classes of WP developers: people working on things that don't touch Gutenberg, who will be writing PHP, and people working on things that do, who will be writing JavaScript/React.
(My suspicion, even though Automattic has denied it strenuously, is that the master plan is to slowly phase out PHP and turn WordPress into a React-only project. So after Gutenberg, we will see more and more components of WP getting rewritten in React, until one day there just won't be any PHP left anymore.)
Thing is, I don't know a lot of PHP developers who are also React whizzes, or vice versa. So I fear that Automattic is taking one of the core reasons for WordPress' success, its absolutely huge developer community, and throwing it overboard. If the plan really is for WP to become a React project, a whole lot of PHP developers are going to need to become React developers in relatively short order. Some will be able to make that transition, but I worry that many will not. Maybe it will pick up enough new React fans to make up for them, I dunno. But that seems like an optimistic read to me.
The more I learn about Gutenberg, the more I wish Automattic had just started a second, React-based project and used that as a clean sheet of paper, rather than taking all these ideas and trying to jam them into WordPress to make some sort of software turducken.