The seed song approach isn't very good.
I think the way Spotify discovery works is looking at songs you have favorited and compares other human created playlists from users that have your favorite songs, then taking songs from those playlists you haven't heard and putting them in a list for you. I think they also do some modeling after that, but it's secondary.
This works really well since there's a human selection element they're taking advantage of and playlists tend to collect stuff that people who have the same taste will probably like.
Radios based on seed songs I've found to be pretty unimpressive, similarly curated playlists that don't know about your tastes are also uninteresting.
It's the combination that works.
The weekly distribution is also good because you get time to explore new stuff each week (rather than just being overwhelmed) and you can further improve things by liking/disliking new stuff.
Good to know the Apple UI improved though.