I stumbled upon this old(er) post while searching for something else and I agree with the Mix GUI idea 100%. I make up most of my playlists on the fly using panes with the occasional help from the search wizard. Currently I find a desire to use the Mix command fairly often, but don't bother because it generally takes me 10 or more minutes to get a proper mix search string put together. I could probably get that time down some if I used it more often, but part of what I love about my setup is being able to build really good playlists from my collection in under a minute.
Aside: Should feature suggestions like this pretty much be thrown over to the MC14 board at this point? I notice that there has not been a single update to MC13 since the first v14 preview was put up.
-JB