Display a warning message when you try to send a song/album/artist that you've already sent to someone.
I share favorite tracks and new finds with a group of people quite often and I can never remember which songs I've sent to them already and which I haven't. I wouldn't want to spam their Spotify inbox with the same song that I've already sent them before, so a warning message would come in handy to let me know not to send it again!
I feel the social side of spotify is somewhat lacking at the moment. Firstly, I can't connect with my Twitter friends, who, let's be honest, have much better music taste than my facebook friends, who are mostly people from my high school that I never spoke to. Also there doesn't seem to be a way to find anybody that's not one of your Facebook friends. There should be a way to search for people who have Spotify accounts, regardless of what other networks they're on. It seems like the only way to find people is to search 'spotify:user:usernamehere' which isn't really the most intuitive, and requires trial and error to guess what their username is.
I think the 'People' tab should show a list of people that you're subscribed to, with a stream of activity showing playlists they've made, albums they've listened to etc. and perhaps the ability to post statuses: recommending songs to your 'followers'. Which brings me to another point. There doesn't seem to be any way of knowing who is subscribed to you (as a user, not just a certain playlist). How many 'subscribers' do I have, and who are they? I guess some of these features may be in there already, but if they are, it's not obvious or easy to find.
I think these features would really help make Spotify more engaging, which would be useful now that Rdio is a competitor in the UK, and does social things very well.
What if Spotify could generate a playlist of the top songs of a user's friends? Right now a user can view trending playlists and a few top tracks but what if you could shuffle more tracks? I know that Facebook can tell which friends interact more/ are "closer." I'm not sure if Spotify has this ability or if it could use this data from Facebook to show users what their "closer" friends are listening to.
Something Spotify is missing is some sort of app or a part of spotify itself that allows users to see how similar their friends musical taste is to theirs. An app that could show this data, and also sort that list of friends by how compatible they are with you or other people would be a great innovation to Spotify. As it is now, the only way to gauge this is to actually look at what songs their friends listen to.
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