There was a suggestion for a track rating, so why not a suggestion to rate a playlist? It is kinda strange this function isn't implemented yet. With this would obviously come a search function for playlist ratings. (Highest Rated under the tab All Playlists?)

My two cents.

Right now when people like a playlist, they bump it i.e. give it a thumbs up - that's our rating system for now. When you choose "All Playlists" or any playlist category or charts you can sort playlists by most bumped. That option is in the right column of the page under "change order." This'll give you an idea of the best playlists on the site.