I really like the idea of a rating filter. Say I only wanted to see art with a certain rating, I would choose the rating I wanted from a drop-down menu, and then search by tags and it would narrow my search by a long shot. I also like the idea of having a recommendation based on what you already look at and favorite. The site would take that into consideration, and recommend art for you based on what you like.

https://e621.net/help/cheatsheet#Searching_Post_Metatags

Panthera said:
longstuff

rating:S
rating:Q
rating:E

in search bar along with other tags.

If you don't want to be caught on a porn site at work, there's e926.net.

Also, the recommendation thing would be useless. It would rely solely on tags, and tagging is so variable that the recommendations would be incoherent.

I don't like to be receommended stuff, I always remove the "Popular in <your country>" entry in YouTube, I hate what my country likes (football, some kind of music and the only two popular channels)

You should follow what TheHuskyK9, CuteDementia and SirAntagonist have said.