Why an acceleration based modification?
We usually think first in terms of a modification at some length scale: galaxies are big, so maybe gravity is different on large scales. This does not work. But there are other scales which are different about galaxies. One of them is the very low centripetal acceleration experienced by stars orbiting within galaxies. This is just as far removed from our laboratories as is the size scale of galaxies. MOND was motivated by two observations: 1) the asymptotic flatness of rotation curves and 2) the slope of the Tully-Fisher relation (M ~ Vflat4).
