I'm just going to scribble all of my many thoughts down in list form:
- Burning Vengeance decks just got a big kick in the nuts.
- The value of Naturalize just went up.
- The value of Ancient Grudge just went way up.
- Havengul Lich now has something to fear.
- Black/White Token decks are effected very little by this.
- Werewolf decks are effected very little by this.
- People may start playing this in their sideboard instead of Witchbane Orb, making "Curse Decks" a potential reality after all.
If you play Innistrad Block, if you're playing a deck that doesn't abuse the graveyard, you're going to want to run 4 copies of Grafdigger's Cage in your main deck and/ore sideboard. There's no question about it. If you play a format were Tormod's Crypt or Bojuka Bog exist, play those cards instead. Those cards are still better.
(Obviously, there are cards which fetch creatures from a player's library. And against those cards, Tormod's Crypt and Bojuka Bog have no effect. In those cases Grafdigger's Cage becomes your go-to guy.)