Albums and items now have flexible attributes. This means that, when you want to store information about your music in the beets database, you're no longer constrained to the set of fields it supports out of the box (title, artist, track, etc.). Instead, you can use any field name you can think of and treat it just like the built-in fields.
For example, you can use the modify command to set a new field on a track:
$ beet modify mood=sexy artist:miguel
and then query your music based on that field:
$ beet ls mood:sunny
or use templates to see the value of the field:
$ beet ls -f '$title: $mood'
While this feature is nifty when used directly with the usual command-line suspects, it's especially useful for plugin authors and for future beets features. Stay tuned for great things built on this flexible attribute infrastructure.
One side effect of this change: queries that include unknown fields will now match nothing instead of everything. So if you type beet ls fieldThatDoesNotExist:foo, beets will now return no results, whereas previous versions would spit out a warning and then list your entire library.
There's more detail than you could ever need on the beets blog.