To answer pat:
There are 189,836 admissible (not "plural") words in our database. Of these 46,899 (25%) are classed as common.
Of course, not all of these words could possibly ever come up in a puzzle. This is especially so since the introduction of the 7-by-many, when I expanded our lexicon to include words of any length. A lot of the very long words would never be playable. Confining the investigation to words of 4 to 9 letters, there are 108,144 words of which 33,647 (31%) are common.
In all the 20,000+ daily puzzles, from 2005 till now, there have been about 76,000 different words playable, of which about 29,000 (38%) were common.