Basically I ask this because of the current behavior of the ignored and interesting filter list. If a post has a tag in both of these lists, it is ignored. It appears from an earlier question that this behavior isn't going to change, but it makes it difficult to filter questions if your interests are fairly narrow.
I would like to propose that a third filter be added, an Uninteresting filter. A Post matching this filter would remain if it also has an Interesting tag, but otherwise be removed or greyed out. The Ignored filter would continue to override either of these.
From a user's perspective, this third filter would be a weaker form of ignoring, and allow conditional ignoring of certain tags. For example, if I was interested in SQL, but not Python per se, I can still see questions with both python and sql tags, but ignore tags with just python, for which I have no strong interest.