My favorite smart lists implement an elobrate genre scheme.
I give the songs very specific genres:
i.e. Regional - French or Classical - Flute, on top of that, I have built a SUPER GENRE smart list that contains items such as classical, regional. Each song have a single classical genre but they may have multiple super genres. i.e.
ROCK
metal
pop-rock
rock
folk-rock
...
POP
pop
pop-rock
...
CLASSICAL
film-classical
classical-flute
classical -> this is for not specified classical music
...
Of course I have an associated view scheme
SUPER GENRE / Genre / Artist / Album ...
In thumbnail view, if you click on ROCK, you see all rock albums. You can go down to see more detail, if you feel you know more what you want to listen.
The cool part is that I let the SUPER GENRES evaluated automatically from specific genres.
I have a Playlist Genre group: SUPER GENRE, in that, playlists such as:
(empty Genre) : [Genre]=[]
CLASSICAL: [Genre]=[Classical],[Classical - Film],...
....
(undescribed Genre): exclude list of the above: CLASSICAL, etc
If a unknown specific Genre is added, it goes to empty Genre, it goes to (undescribed Genre). In that case I just updated the related SUPER GENRE playlists to contain this genre. If I add a new super genre, I have to update the smartlist (undescribed Genre) (occurs rarely). If a song is added that has no Genre, it goes to empty genre, when I add a specific genre it goes either to (undescribed genre) or to the relevant playlists.