For artists: Right now, there are three tabs on the "Artist Property" dialog box (General, Statistics, Web Pages). At least two more would be useful:
Notes--to add biographical information, etc.
Images--to add images, and preferably more than one.
For disks: Only one image is allowed per disk. Why not allow for more? It would be great to add an image of the back of the CD, and also the spine (makes it easier to spot the CD when looking at a full shelf).