Whenever any observations are excluded from a regression analysis (lm, glm, and
other similar procedures) because of missing values, I would find it very useful
if this fact is directly visible from the output. I think that the information
should not only be available (I can e.g. look at length of the na.action element
of the lm object) but that a serious statistical software should draw users'
attention to the fact that observations have been excluded.
For convenience, it would also be nice in general if the number of observations
used in the analysis is indicated (for lm it is of course possible but a bit
awkward to find this number in case of many parameters).
I hope that this will be implemented because it is quite easy to do (as far as I
can see). It would make it easier for students and applied researchers to comply
with my preaching to always report on the number of valid observations and the
portion of values excluded for missingness.