The tag filters right now get questions through that have any of the tags in the filter. (for example here's mine)
It would be really helpful to support combination of tags where the question goes through only if it has all the tags in that combination.
Take for example generics on SO. It contains every single language that has generics which makes it pretty unusable as a filter. But combined with a language tag like C# would be extremely useful (like you can do on the site itself).