And then there are the VFX. The revisionist CGI effects have dated rather poorly. It would be one thing if the original VFX could be recomposited, as TNG's were, but the elements are mostly long-gone.
So, 1960s and/or 2006 VFX upscaled to 4K, or from-scratch 4K renders. And, frankly, I'm not a fan of tossing the old VFX out in favor of the Okudas' fannish and arrogant revisionism.
I think there's a case to be made that TOS is best experienced on VHS or DVD, frankly. The show wasn't made to stand up to HD or 4K scrutiny. I like the look of the Blu-Rays, though.