I do tag books that I've read in my library with descriptions about the books subject. I also tag books that are in my library that I want to read as "wishlist". I don't really think we need standardized tags because there are so many tag options that someone may think of a ag for a book that no one else would, but is still a good tag.
My top 5 tags are:
read years ago - This indicates to me that I didn't read the book in 2007 or 2008.
wishlist - Books on my list to buy and read
read in 2007 - Self explanatory
own? - Books I have read but I can't find in my library
motherhood - Books about life as a mother