Add grouping options to filter criteria (parenthesis around and/or expressions). That way one can build expressions, for example, to filter out certain emitters for certain log levels only:
( "Emitter" NOT equals "some.uninteresting.emitter" AND "Emitter" NOT equals "some.other.uninteresting.emitter" ) OR "Level" is or is more critical than "WARNING"
You are right, this is not possible at this time. We would also like to allow parenthesis, but the solutions we have found to do so did not look good enough for us:
allowing free text expressions (like the one you gave in example), which could lead to many syntax error (and less user-friendly),
and a graphical support that would make the UI much more complicated (buttons to add/remove parenthesis before/after a filter).
If you have an idea to allow this in a simple way, we would be happy that you share it with us