I swear there must have been some changes to the alloy or the planchets have gone through different chemical bath treatment.
Early nickel copper coins struck before 1980s have a duller grey look whereas post 1990s have a much brighter polish look. I've seen some worn out post 2000s coins but they still have a different overall look compared to some XF+ coins that I find in circulation.
My partial coin collection http://www.omnicoin.com/collection/gxseries
My numismatics articles and collection: http://www.gxseries.com/numis/numis_index.htm
Regularly updated at least once a month.