Mass -- F-Line gives you a tremendous amount of background on the history -- but doesn't really answer the questions -- "the entrance area is nice and new, while the platform area is not quite as nice."
The Entrance area is a brand new replacement for the aging "temorary station head house" and hence has escalators and elevators plus glass --- the rest of Charles/ MGH was just a renovation and restoration of the original (circa 1920's) platform enclosure (where threre is a roof and walls) and the much later (circa 1990) platform extension on the Longfelllow to enable 6 car trains
i never understood why a roof couldn't have been provided for the platform extension -- indeed if I was doing the rebuild -- I'd have put a plexiglass shield over the whole platform including the tracks to give the passengers a much more pleasant waoting area in rain and snow -- however for some reason just like Science Park -- the rebuild stopped short of improving comfort for the waiting passengers