Here are just a few of the wonders you can expect when you log onto WoW after updating.
Your talents are all reset
None of your addons will work correctly
Your spells don’t do what they did before.
Stat changes are in place (no more spell hit or defense, etc)
Your gems are all different (and will almost certainly need to be replaced)
I must confess that I am excited for Cataclysm, and I am sure that the undertaking of changing the entire WoW world is far larger then I can even imagine, but does every patch require me to update, redo, or reinstall everything that helps me play the game?