I thought there was already a ticket for this, but I can't find it.

In mason, it was possible to make a list of artists - http://classic.musicbrainz.org/show/collection/artists.html
and set which types of releases you wanted to see - http://classic.musicbrainz.org/user/collectionpreferences.html
and then see which matching releases from those artists you didn't have in your collection - http://classic.musicbrainz.org/show/collection/missing.html
This still hasn't be implemented in NGS.

Some Trac tickets about the feature:
http://bugs.musicbrainz.org/ticket/4001 - Missing releases doesn't show Various Artist releases
http://bugs.musicbrainz.org/ticket/4020 - Easy way to see all missing releases for artists in your collection
http://bugs.musicbrainz.org/ticket/4135 - Missing Releases isn't paged
http://bugs.musicbrainz.org/ticket/4033 - Missing releases is not differentiating between different releases with same name