You can read about some of the new features on the Spotify Blog. I'll go into a little more detail below:
Tracks that are intended to play continuously, with no gap, will now do so. Where there are silences between tracks on an album, they will be as long as intended to be. In short, albums will now play as intended (as they would do on a CD). It worth noting that "gapless playback" doesn't mean that all tracks in your playlists will play with no gaps between them.
Tracks will play back gaplessly when
played from an Album page
played from playlists or queue, where any pair of sequential tracks from an album appear
Since this is an important feature where it applies, and doesn't really detriment play from randomly composed playlists, the feature is turned on by default.
This one explains itself. Try it out.
You'll find these features in Preferences (Win: Edit > Preferences | Mac: Spotify > Preferences )
Offline mode (Premium only)
You can now choose to go offline in the desktop application, just as you can in the mobile apps. Some customers have asked for this. It means that you can listen to offline synched playlists and imported local music files without any P2P complications that can sometimes arise if you are, for instance, using a corporate network, or using a mobile 3G dongle.
You'll find the setting under the File menu in Windows, and under the Spotify menu in Mac.
Hide purchase ("Get") buttons (Premium only)
Many of our subscribers are happy with having all of your music in Spotify's catalogue, and don't feel the need to regularly buy music to "own". These buttons are visual clutter. You can now hide them.
If you do feel like buying a track, or re-downloading a track that you have already bought, you can
show the buttons again, or
right-click any track, selection of tracks, etc
Playlists - View as album list
This isn't actually a new feature, but it was previously hidden behind the keyboard command, Ctrl+G. Now it's on the View menu too. Use this command while viewing a list, and it will show you album art thumbnails (Repeat the command, to go back to the basic view). This is available in all lists, apart from Search results, Inbox and Play queue.
Local files are aren't yet supported for gapless playback. The status on that one is "Working on it".
The entire catalogue has not yet been re-encoded for gapless playback. The tracks are being re-encoded in order of popularity. This means that practically all Spotify "plays" will be gapless-enabled. You may still find some music that does not play gaplessly, if you have exquisitely obscure taste. Please don't report any such tracks for the time being.