I am not sure that we are really talking of (all) the same things
First, I'm not going "layout first" at all. I am experimenting to migrate to hugo several existing sites, quite different from each other, each with a lot of content and its own requirements in terms to "what to convey to visitors".
Second, I am talking of things being totally broken when you move from one theme to the other, not of some parameters not showing. If I thought Wordpress is better than Hugo for my needs I wouldn't be here. But on wordpress, if I configure a blog to use a theme and tweak that theme, then switch to a different one, the site is still usable. Maybe ugly, but fully usable.
Whereas with Hugo, just to make two examples:
if I start with one theme, copy its "single.html" template to layouts/default to tweak ONE parameter (in my case: add links to translations), then try to load another theme, then hugo throws a lots of errors, and doesn't compile the site at all. Part of those errors are because partials that do the same thing (i.e. render the footer) have different names in the two themes.The only way I see to avoid this when testing a different theme, is to rename the layouts folder to something else, e.g. layouts.academic, then repopulate from scratch an empty layouts folder, one file at a time. Is this correct?
if I find a theme that I think it's perfect for me, except one non-trivial thing, it is far from obvious how to add that feature, even if it is something surely supported by Hugo.
Concrete examples of point 2:
a. where do you paste, and how do you modify, the code in https://gohugo.io/extras/menus#rendering-menus to make the aglaus theme display a menu with what you put in .Site.Menus.main. ??? Yesterday, at some point, I tried to copy that menu code referenced below in (IIRC) the single.html template of the aglaus theme: I got no errors and.. totally blank pages, except the sidebar...
b. just for fun: if I wanted to add a sidebar to aglaus, where should I write the corresponding partial?
I'm not asking for a point and click platform, mind you! I'm saying that a less unpredictable behaviour (see the links to translations case), as well as concrete examples in the docs of how to do the things above, wouldn't hurt at all.
One thing I really don't understand, instead, is the fact that several themes seem only installable via git. This is not a problem for me at all, but not providing a single tar/zip file makes that theme unreachable for many potential hugo users. I know it's up to the single theme developers, but I wouldn't feature in the official theme gallery themes accessible only via git. Or at least I would flag them explicitly as "only usable with git" so users without git wouldn't waste time looking at them