Almost with the same criteria you use with the added category of quality

Quality is a given for me, not a category.

I also base the same criteria on photos posted here as well, considering that photography is and always will be a form of art

I often consider what my recent favs will look like on my user page, that they can serve as advertisement of artists I like to those rare visitors to my page, so I won't often +fav a lot of stuff by the same artist at once, and I might fav something I only sort of like if it's from an artist who doesn't post that often.

As to criteria, I really don't have any firm set. I just like something, deliberate a little, then +fav.

It would seem to me that you do have certain criteria. Quite strict ones, in fact. It's just that yours aren't based on the subject matter, but more the artist who produced the piece.