Let's list all of the addons blizz has integrated into the game
Power auras - I think this is one of the best combat changes blizzard has made. With unique and colorful displays for each ability and good positioning on screen, I am very glad blizzard implemented this.
Armor sets - while not as detailed as some Addons this is a great addition as it uses no memory and does it's job simply which is all most people are looking for.
Voice chat - while vent/TS isnt an add-on technically it was a nice try. Needs alot of work.
Questhelper - again not as detailed as third pArty Addons but it's always worked for me and once again it uses no additional memory and ifi recall questhelper was a memory hog.
Dungeon maps - while not fully implimented for all instances and raids it is great for those confusing instances (hello WC BRD BRS).

Feel free to elaborate and add what I missed!

---------- Post added 2010-10-15 at 12:09 AM ----------

Just remembered the arena add-on Proximo was added as standard