In terms of the "Why Mavericks?" section, >I knew I wanted an operating system from before Apple abandoned the Aqua design language. I suppose it depends on your definition, but that likely does mean Mavericks is the latest available. For my money though, El Capitan (10.11 to Mavericks' 10.9) was the local maxima (speed, stability, capability). I've no inkling what issues using that would entail—I had no idea that Mo…
Strictly speaking, Mavericks (and Mountain Lion and Lion before it) were already some way through abandoning Aqua. Lion dropped the beautiful blue scroll bars that previous OS Xs had, replaced the pill-shaped buttons with rounded rectangles, and somewhat flattened the overall UI as well, though not to the extent that Yosemite did.
I do miss Snow Leopard's scroll bars though, as I explicitly call out on the website!