This is great - thank you for writing it up! As the person who built the system, I've spent a lot of time thinking about how to find a balance between maximum functionality and an interface that's not overwhelming or confusing, and I agree that there's definitely still a lot of room for improvement. Just the help info is a lot more text than most people will necessarily read or remember, so how to work that in can be tricky. (I'm really liking what Github did with the placeholder text on their advanced search: https://github.com/search/advanced) And integrating all the complexity of the tag system with the search engine is a bit of a challenge, although we do have the capacity to do excludes based on synonyms and metatags, we just need the time to work that into the interface.

The feature I actually use the most as a reader is advanced searching for date less than x days ago (whenever I last checked the fandom), in a particular fandom, sorted by kudos, comments, or bookmarks. I find that really helpful for following big, active fandoms where you may not have time to look at everything as it's posted but you want to make sure you don't miss the good stuff. (Obligatory caveat here about how the best works may or may not be the ones with the most kudos/comments.)