Year Tag
A tracks 'year' tag is editable in iTunes and I use this field for the release year of all my tracks. Personally I like that its a 4 digit tag as month and day are not important to me, but when using Traktor it appears isn't available to be shown as a tag column in the library.
It turns out Traktor calls the field 'Release Date' and it adds and month and day of 01/01 to the year value. The catch is though, at least currently, it is only shown for a Traktor playlist. When viewing iTunes playlists the release date field is shown as –/–/–. If you drag a track out of an iTunes playlist into a Traktor playlist the field is shown no problem. Hopefully this is something that will be fixed in Traktor in due course…