Unless the US version of Spotify is drastically different from the European version, discovery on Spotify is excellent in my opinion. The Radio feature does exist (although it's not one of Spotify's strongest features) and when you're on an artist page there is a tab to view related artists. There is even an radio tab on the artist's page which will play music from related artists.
That's a shame for now, but the fact that related artists are presented very clearly (and you're able to quickly see their most popular songs) doesn't make discovery as bad as the article is claiming. I find myself listening to a lot more different music with Spotify than other discovery services such as Last.fm.
Amounts to browsing random peoples' music. Compared to MOG's discovery features, MOG's is much better. You might get lucky if a friend just happens to find lots of new music you like though.
