really.... why not just keep radio and doctor? Why is that so radical?
Now there's a fine suggestion! I would prefer this by far. I'm not sure if there is any point to referring to the pulled build which had some changes to Play Doctor, but in build 44 the big problem I could see was that skipping a song only had an effect on the end of the playlist, 100 songs later. I also noticed that the smartlist-type rules you enter don't persist - another serious step backwards for me compared to Radio. For me to be able to use PD the same way I use Radio, I would have to enter the same set of rules manually every time I opened the program (or changed Zone, from what rick.ca said).
It probably wouldn't work this way, but if it does make that arbitrary choice by somehow generating a search term in lieu of one entered by the user, then it would be interesting to know what that term is. If it were shown in the search box when the list was stopped, it might give the user an idea for a different or better term to use for the next request.
I can't remember what I read about how Radio determines what to choose next, but I believe it uses a mixture of the obvious candidates like genre and artist as well as things like bpm and intensity. In any case, for PD to operate like/better than Radio, on user pressing play with no terms, it would arbitrarily choose a track and then generate the rest of the playlist using the user's variety choice to determine how far from that original track the rest of the list deviates, in terms of those fields. However therein lies the problem. With Radio, if you didn't like a track you could just skip it - with PD if you don't like the track choice, depending on your Variety setting you could find you don't like/aren't in the mood for most of the playlist. Then the only thing you can do is press stop and start again until you get what you want.
Here is my version of an evolution of Radio:
The ability to specify additional fields which Radio should use when determining what to choose next (instrumentation, record label, era, etc.)
The ability to save the Radio state, so that if I've been really enjoying a particular list but have to restart my computer, I can simply take up where I left off.
The ability to simply toggle radio on or off, without having to restart the playlist - if I'm listening to something specific, then want to use that as the basis to start a Radio session, being able to simply toggle radio on would be great (just like the shuffle, loop controls), rather than having to stop playback and use the menu and click through to get back to where I was.
A graphic input on the Playing Now file-list to allow a simple 'more like this' toggle for each track. Skipping essentially says 'fewer like this', so a positive version would be excellent.