The stop sign symbol implies that Picard isn’t able to save the information for some reason (but that the files have loaded ok).
Are you sure that the drive isn’t write protected, the files aren’t set to read-only, and they aren’t being used by another program? Those are some of the most common reasons for that error.
re. Musicbrainz taking a lot of effort, that’s because it’s a very versatile and comprehensive tagging tool and database. Nobody said retagging all your files with MB would be quick (or recommended it?) unfortunately! iTunes will happily crush all of your music to fit its database in seconds, at the click of a button. There’ll be a lot of mistakes and data lost - not that that’s a problem for everybody, most people don’t care as long as it works and it’s as convenient as possible. MusicBrainz really serves a different kind of audience.