As a start, correct ID3 tags and an album cover bigger than the size of a thumbnail would be great. Bonus points for configurable folder and file naming scheme and playlists generation upon download.
I always felt kinda stupid paying 10 Euros for an album, then spending 10 minutes fixing metadata and downloading a halfway decent cover image from discogs.
You'd get better data quality from filesharing sources, since fans obviously care more about releases than labels. Same goes for movie releases btw.
I think that's because buying a digital album has so far been really unrewarding.
Artists need to come up with a package for their music that people actually want to buy.