It seems like users are being forced onto the 'Open' player from the play.spotify.com player. There are a number of issues with the new 'Open':
Check-mark support is missing
Not designed for landscape displays (wastes a lot of space)
Icons are way too big on a desktop display
Artists tab doesn't allow playing all check-marked songs from an artist across all albums.
There are probably many other features that people have got used to in the old 'Play' player that are also missing in the new 'Open' player. These are just the ones that affect me the most. Please either make the 'Open' player the same as the 'Play' player, or continue allowing access to the old player.
Right now I think I'd have to search out all my favourite tracks again and create playlists, but this is a huge hassle. I will try the IE10 user-agent hack for now, but if that stops working, what is there to keep me on Spotify? I can't use the Desktop version here.
Spotify is always testing this and working on improving Spotify. You can post your feedback and ideas on the Ideas board. Be sure to search for it first! Let me know if you have any questions.