Track Info displays (aside from the obvious info) the iTunes current track’s name, artist, rating (including ½ star ratings), comments and track equaliser. If set manually, the track’s start and finish times are also displayed.
Displaying track artwork is possible thanks to this code by Nick Fagerlund, which takes a track’s raw image data and converts it into a TIFF file to allow Growl to display it. Customising the script to include any other iTunes information would be fairly straightforward (the code contains some explanation notes).
In Quicksilver: Track Info.scpt ⇥ Run ⌃⏎ Add Trigger… ⇥ HotKey ⏎ – and you’re good to go.