When editing a smart playlist one can only choose between "all" (AND) and "any" (OR) for the types of matching condition relations. It would be nice to be able to add more advanced algorithms for matching.

Example (what I tried to do):
.."Genre" is "Rock"
AND
...."Rating" is 0 (unrated)
..OR
...."Rating" is at least 2

When I realized that the smart playlist interface wouldn't let me do this I tried modifying banshee's database (banshee.db) directly. So I entered (in compliance of how the other smart playlist entries where written...) the following in the "CoreSmartPlaylists" "Conditions" colom as a one-liner:
<request>
	<query bashee-version="1">
		<and>
			<equals>
				<field name="genre" />
				<string>Rock</string>
			</equals>
			
			<or>
				<equals>
					<field name="rating" />
					<int>0</int>
				</equals>
				
				<greaterThanEquals>
					<field name="rating" />
					<int>2</int>
				</greaterThanEquals>
			</or>
		</and>
	</query>
</request>

When I started banshee afterwards I selected the playlist and banshee showed me all entries form my library instead of just the expected results... :( Also everything was reset in the playlist settings dialog. (No surprise there...)

Conclusion: Please add at least the possibility to add more advanced matching algorithms in database and maybe (on day) also to the interface...