I don't know about you, but I run a ton of add-ons. I run add-ons for fighting, I run add-ons for rep tracking, I run an add-on for auto-selling grey items and repairing when I go to a vendor. Blizzard, in the past, has taken some of the favorite add-ons and rolled them into the game (loot window follows to mouse, integrated quest map).
One thing that has always bugged me was the "looking for" interface. When out on my warlock in old zones, I like to search for 'low-level quests' in addition to pick herbs. On the current interface, this is an either/or option. You can't do both, while logically, my orc could.
If you could provide input into the game, what would you have added to the Cataclysm interface?