Easy way to see changes between core sets?
wiki.mtgsalvation.com is your friend.

http://wiki.mtgsalvation.com/article/Magic_2012/Changes, for instance, has complete lists of all the cards that were added to M12 and removed from M11.  You can find the same information for all previous core sets too.

In addition, there are useful sections on changes in rarity from previous printings, and also a running tally of cards that had (up to that date) been included in every Core Set.  Go go Giant Spider!