the question is:
How do you know what constitutes 'Downloaded"?
Some of it comes from the book (and some is pretty sparse

)
Not all sites provide the same metadata fields (see the tick boxes in the Configure Metadata sources )
searching on
<the_fieldname>:false , will tell you if it is empty

The Quality check Plugin has many checks to help find and 'normalize' metadata entries
The real question: What do you consider
Valid Metadata?
GIGO
40 (mostly redundant) Tags?
Sales Hype (comments) instead of a clear synopsis of the work?
A Sample chapter in the Comments?