The general picture we have of disk galaxy formation is of cool gas settling into a disk in a potential well dominated by dark matter. The dark matter is assumed to reside in a dissipationless, effectively spherical distribution, and to interact with the potentially luminous (``baryonic'') matter only gravitationally. To make predictions from this general picture, we need to specify something about both the halos and the baryons. I would like to emphasize the importance of making a priori predictions. If we simply tune our favorite model to fit to the data, we will certainly fit the data.
At minimum, two parameters are required to describe halos, such as a
characteristic mass 
h and size Rh. Further
parameters describing
axial and kinematic isotropy, etc., could be invoked, but these distract from
the basic point and are not fundamental. For the baryons, many parameters
might in principle come into play: initial angular momentum and gas
temperature, conversion of gas into stars and feedback into the ISM,
and so on. It would be nice to study a population of galaxies for which
the effects of the baryons were minimized.
Low surface brightness (LSB) galaxies, with central surface brightnesses
µ0 > 23 B mag.arcsec-2, turn
out to be just such objects. They occupy a different region of
parameter space than do high surface brightness
(HSB) spirals, so they provide genuinely
new tests of ideas contrived to explain observations of HSB disks.