Thank you for your feedback.
> The decision of what does and doesn't go in the sidebar is very non-intuitive
We try to put top level navigation into sidebar and everything else in the main page. For example navigation to `Projects` is in sidebar however tabs to switch between starred projects or personal projects is in main body.
> For example, in groups and user profiles you can find the list of projects in the main body of the page (and it's not visible immediately). Why is it not in the sidebar if you have a sidebar?
Good example. Agree its still non-intuitive in some places. For you particular example we created an issue to fix - https://gitlab.com/gitlab-org/gitlab-ce/issues/13480.
> And then the ordering is also a bit weird -- I don't think many people would consider the order in GitLab to be "in order of importance" or even "regular use"
Ordering of projects is either by last activity or alphabetically. But in any case there is usually a dropdown in UI to sort by other criteria.
> And why can't I see the set of files in the repo when I first open it?
You can set what to see first at https://gitlab.com/profile/preferences page. We believe README is something people would like to see first we allow users to set different default page based on their preferences.
P.S. Thank you very much for your feedback. It will help us make GitLab UI more intuitive.