The quick menu is accessible with Spacebar and provides a way to discover and call any available command. Shortcuts are not necessary for discoverability.
However, shortcuts are well-configured and when using pie menus, you only need to know a handful of them to get started.
> If you don't, then you have to click the drop down mode selection menu, which is one of the dozens of widgets on screen
Tab is one of the first (and only) shortcuts you need to know to get started. The same options can still be accessed from the quick menu however. I wouldn't call this a discoverability issue.
> what the hell is 'Grease Pencil'?
Ask the documentation.
> when in the UV workspace, there's literally nothing in the interface window except the vertices/edges. How do I select all vertices? How do I select multiple via box? How do I do numeric input?
With the same exact gestures used to select vertices in edit mode. Blender's UI is highly consistent these days. Numeric input is as easy as calling an action and then typing in the number before confirming with Enter.
> The defaults are also terrible - eg click to set 3D cursor, instead of select.
I wouldn't call this terrible, just different-- and besides, Blender Foundation has just recently made the cursor defaults reflect the common defaults found in other 3D modeling packages.
> Overall, Blender seems to suffer from a classic case of open-source programmer driven development.
Have you used Blender in the last 7 or so years? These seem like antiquated criticisms. The Blender UX is vastly different and much more focused these days.
Every modeling suite has a learning curve. I'm interested in what Blender is sorely lacking. Blender's discoverability is actually very good and Blender itself might be my favorite example of how to design functional application UX. This wasn't true a decade ago but it's true now.