My understand is that the main vector for tag lookup is manual searches based on metadata driven from applications such as Picard. Accordingly, this should be a relatively easy to use search mechanism (user should not need to know advanced query syntax) and should maximise knowledge of which search term belongs to which field.

In MBS-1520 this was changed to use the search server which appears to be giving better results (not better columns, alas) however it's really not friendly for regular users.

e.g. http://musicbrainz.org/taglookup?artist=morning+musume&release=first+time&track=ai+no+tane

The nature of manual tag lookup is that you may need to tweak your search; correct a typo; try a slightly different release name. However the way the complex search query is constructed makes it impossible to do so.

I suggest hiding this from the user. Display the search results widget as it currently is, but display the regular taglookup fields; pre-filled with whatever the base query was at the bottom of the screen rather than the complex search query box. They don't need to know what nasty complex search query was generated, just the source fields that were used. (similar to the old taglookup at http://classic.musicbrainz.org/taglookup.html?artist=morning+musume&release=first+time&track=ai+no+tane )