The rounded corners thing is definitely all about individual taste. my brain just likes to have the whole space filled. The rounded corners for me feels so.... 2000's Windows XP/ Early GNOME.

The start menu thing, yeah you can configure it. And no, before Windows 8, I think, there used to be a Gao of a few pixels around the start button if I remember well.

And as for the start menu tiles, again that's all individual taste I guess, but I sincerely think that ergonomically speaking, it is better.