The function "Play top songs" for artists does not include all songs listed on the respective Last.FM page. The reason is that the titles on my server sometimes deviate a little from those in the Last.FM list.
An easy first step would be to make the matching for this function case insensitive. A slightly more sophisticated additional approach would be to use something like a similarity metric like the Levenshtein Distance (Edit Distance) with a low threshold for artist names and a slightly higher one for song titles.
Since the connection to the Last.FM API is already implemented, it could be further used to import or use playlists on the fly from Last.FM (or even other services like playlists.net), adding or playing only those that can be found on the server (with the same similarity metric used). This would greatly improve the usablity of Subsonic in my opinion, since it almost takes too much effort to create playlists manually for every occation or mood.