I'd also like to echo that this would be a useful feature, especially for larger game collections. As a recent example:
- I check out a lot of game demos during every Steam Next Fest
- I tag them all as "demo"
- the demos I like get an additional "demo - liked" tag, as a hint to possibly check out the full game in the future
- now when I filter for the tag "demo", it will include all demos, those with just a "demo" tag aswell as those with the extra tag "demo - liked"
- but what if I just want to display the demos that I didn't like (without the "demo - liked" tag)?
- an exclusion filter option would make it possible: I could simply exclude the "demo - liked" tag so only pure "demo" games will be displayed