When I first started listening to Spotify (about a year and a half ago), I found the dicovery really high-quality, but it just seems to have gotten worse and worse. Discover Weekly is particularly bad. I find that the first 5 or so songs are okay, but from there is rapidly decends into madness.
I have the weird habit (though I'm weening off) of keeping all the music I listen to out of curiosity on a big playlist together, whether I liked it or not, as a way of keeping track. I thought that might be throwing Spotify's algorithm off (and it probably still is), but it seems it's definitely not the only reason.
I've decided to just trust that Last.fm is doing it's juob and not add any more to that playlist.