In a similar vein to http://bugs.musicbrainz.org/ticket/614 and http://bugs.musicbrainz.org/ticket/1197, Picard really needs a more advanced view to compare metadata than the simple panes at the bottom of the screen which only show the most basic of fields.
As people become more concerned with tagging additional fields (genres, disc numbers, original release dates, for example) this is probably going to become more important.
One option within the existing UI design may be to give the "Details" tab for matched tracks on the right-hand side some sort of diff view (two columns), at least for the metadata that Picard knows about.
Alternatively, perhaps the bottom old/new pane could expand/contract into "more fields" vs "less fields", at the cost of taking lots of real estate and maybe being a bit ugly. The latter at least would allow easy keyboard cycling between tracks in releases to eyeball the diffs.