The other thing that you get with the Addon's is like The Sims, the more addon's you get the more goodies you get, and if you play online some Servers require you to have X addon.
Personally for me, the Epic Levels, and finally the inclusion of Wizard's Wardrobe, along with other such nifty ideas like sky, will make me pick this up tomorrow.
As it was mentioned in a review the only thing I'd want more is more "Casting Voices" to go along with the new spells.
Now Bioware.. start on that KOTOR addon or Sequel, and this time add the damn Editior with it so we can have more fun once the game is over. After playing it 3 times through on Xbox I don't want to even touch it again.