Thanks @davidakennedy!
The default button style should be round, round enough to be noticeable. Most default themes aren’t that way.
Correct! It's even called 'rounded' in the editor UI (the options are "Rounded", "Squared" and "Outline")
If we changed the button styles now, it would be only for Gutenberg buttons, not all buttons in the themes' designs, right?
Yes -- and it would just be specifically for the button block, so changes wouldn't even affect other blocks that have buttons, like the file block.
Although, I'm not thrilled about changing designs once it's out there, I think if we could scope it to Gutenberg-only block buttons, that would be okay. I'd rather have things match consistently than not for people.
This is the way I feel, too :) I don't like the idea of it changing now, but at the same time, the controls don't really make any sense -- for most of the default themes, the buttons have the slightest border radius, barely distinguishable from the "Squared" option. So you have two options that pretty much look the same.
Since the CSS changes will be minimal across all the themes, I'm going to update this ticket cover them all, and will circle back with some screenshots.