I think installing a game feels like a commitment. If all I need to do is open up the browser and the game will start playing I can just check it out without making that commitment, it will get me to try it if I'm undecided, if I don't like it I can just close the window and be done with it.

You mention Spotify, I think Pandora is a great example of overcoming that feeling of commitment. If I go to the Pandora website I can listen to music right away by just typing in an artist. If I go to spotify I need to download some client and who knows how hard it will be to get rid of. If I complete the downloading step then I need to sign up for the service (I know they changed it to facebook login recently so I don't know if it's the same) just to start listening. I don't even know if I will like the service at this point because I haven't even got to try it yet. I think I would describe this as friction.