This wiki page has a list of the main emotion tags on the site: https://e621.net/wiki_pages/8953#emotions

I noticed that in addition to the basic emotion tags, other expression tags exist and are in use:

• angry has angry_look, angry_expression, angry_face, angry_eyes and angry_eyebrows.
• worried has worried_look, worried_expression, worried_face
• bored has bored_expression and bored_look
• annoyed has annoyed expression

happy and sad don't have as many. The happy_look tag is on a few images. happy_face is aliased to happy, and sad_face is aliased to sad.

Is there any reason to have all these facial expression tags in use, alongside the basic emotion tag? If not, should they be aliased to the basic expression, in the same way as happy and sad?

There's also surprise, surprised_expression, surprised_face, surprised_look, and surprised_eyes (with surprised being aliased to surprise). There's definitely an issue here as these tags are pretty much all for the same thing. Ironically except surprise, which has the alias from surprised, as it can refer to the act of trying to surprise someone but failing to make them look surprised.

I'd think angry_expression, worried_expression, bored_expression, annoyed_expression, and surprised_expression would be the proper tags, since it's the expression being tagged, while the "base" tag can be taken for a mood or action that may or may not be directly visible. e.g. muted desaturated colors with a face looking into the distance giving a sense of sadness, but there not actually being anything sad in the picture.