> What specifically? Just comparing Nikon mirrorless to Nikon DSLRs, the F-mount (for DSLRs) has, for example, the highly regarded 14-24mm f/2.8. What's the mirrorless equivalent to that?

The old 14-24 is a mediocre lens compared new ultra wide zooms. Nikon hasn't got to an z-mount f2.8 yet, but their f4 is already sharper. When they do a f2.8 it likely will be even better.