I often use the Keywords-tag to find certain themes of music. It's great that the auto-tagging feature can get these keywords from the Last.fm tag cloud, yet a big part of my music remains untagged..
When auto-tagging, a lot of tracks with unique Album-tags (Chart names, ect.) aren't being found on Last.FM.
What I currently do is remove the album tag, reuse Keyword tagging and place the old Album-tag back.
It would be great to be able to chose whether or not MB sends the Album-tag or an empty Album-tag to Last.FM.
Or, when it doesn't find tags with the original Album-tag it could try again with an empty value of tag.