What it doesn't have is a decent radio feature that set Last.fm and Pandora apart; there are 19 broad genres and you can select decades, but that's it; so as a tool for discovering music it's not all that great; because either you listen to a very broad range of music, or you manually search and select tracks yourself.
ETA: There is a radio function, I just misunderstood.
OR you use the greatest tool in Spotify's repertoire; the reason I heard about it and the reason it may well catch on; the sharing of playlists. When a user creates a playlist from the music in Spotify's rapidly growing library, that playlist can then be given am http link and made available to anyone the link is shared with, either as listeners or collaborators.
As my playlists tend to be fandom or RP-based, you can bet your arse that people I play with a lot are going to be bothered into collaborating on playlists as well.
Meanwhile, I'm copying my iTunes playlists onto Spotify, and realising that I never did share on here the Being Human playlist I made.
The soundtrack was one of the very first reasons I fell in love with that show; like Supernatural and American Werewolf in London, it picked the songs on the soundtrack carefully for the lyrical and emotional relevance to the show, and has very much influenced my impression of the characters (Mitchell, for example, having Johnny Cash and Ivor Novello in his record collection, George transforming to When the Sun Goes Down, and I'll always associate the Eels' Friendly Ghost with Annie)
Anyway, in a fit of ridiculous unoriginality, I referred to the BBC's Being Human Blog in order to construct a playlist of all the songs used in the TV show. Spotify turned up most of them, with the exception of Marc and the Mambas and hymns Michael Crawford's On Eagles Wings Album. For the two songs from that album, I substituted what I considered suitable alternative versions of the songs, but In My Room is missing until Spotify manages to include it.
The entire playlist of forty songs can be listened to, using Spotify here:
ETA 2011 The playlist has now been updated, revised and expanded to include all songs up to Season Three Episode 6 using the BBC's soundtrack listing. Thank you to everyone who has joined the playlist!