Having just encountered some problems with labels (already reported as https://bugs.kde.org/show_bug.cgi?id=140542), I think that one useful feature for advanced users would be the ability to specify an SQL expression whose results are the smart playlist. From my understanding of the comments on this bug, such statements are already prepared internally from the user-friendly interface. Adding SQL expressions would allow one to use wildcard matching or construct queries that are currently impossible to represent in the user interface (it's hard to find ones that make a lot of sense, but [time < 1 minute OR time > 10 minutes] AND [label = foo OR label = bar] is impossible with the current interface, even if the above bug were resolved).

I am guessing this feature would have to wait for 2.0 because of the feature freeze mentioned in the forum. Is it possible to implement this in a script? I would really like to be able to use before the release of 2.x