It looks like filters in Discovery are all logical AND filters. This makes it impossible to build more complex queries -- for example, to find all discovery items which are incomplete because they're missing (say) a goal OR an effort rating OR a description.
I also can't seem to find a way to do NOTs -- for example, I may want to see all items which are NOT archived, or which are NOT "done". You can get around this by laboriously choosing all the other options, but this is inflexible and slow.
To be honest, it would be great if we could just pop a JQL query in for the filters -- and perhaps have a hidden magical "PROJECT = XX AND ( ... )" tacked on. 