My top 4 features I'd love to have in Canola :

1) zoom slider in full screen picture mode (as mentioned earlier)
2) gapless playback (already talked about, but a BIG +1 on that)
3) automatic thumbnails generation for videos
4) UPNP implementation

Also, for album covers, couldn't the python script be implemented as a plug-in ? Wouldn't that shield you from legal ramblings if you don't provide it with Canola but the end user has the ability to run python scripts as plug-ins ? That script is perfect for me otherwise, as I don't have most of the album covers for my rips. It's far from user-friendly but I guess it will work for now.