One of the things I like is that you can choose the codec and the quality in which you get the music. Sometimes (not often) you can even get it in lossless formats. And you pay for the quality/bitrate you want (even with lossless it's still very cheap). And there is no DRM (you are free to share it and move it as much as you want).
I'd like to push the Wavpack and TTA codec as they are both good lossless formats, open-source, cross-platform (Windows, Linux, OS X) and CoreCodec supported...
I'd also like to know if it would be possible to get their database info in XML format to be used inside CoreLib.
And finally as I download the music I buy in the best quality I can FLAC > AAC Audiophile > MP3, I'll need a tool to reencode the content for my portable jukebox (as long as I don't have one that play all these) and preserve the tags... Maybe GStreamer on Win32 could be a good example of use.