Yes you do
Officially, you will need to purchase Cataclysm in order to learn the ability to fly in Kalimdor and Azeroth. However, there seemes to be a loophole that if you try the Cataclysm trial and learn this ability, it will stay learned even if you do not buy Cataclysm itself. This of course is in itself a bug, and any abuse of it may be punished or reverted by Blizzard - if it hasn't been already.
Depends on what kind of addons they are. You don't need Cataclysm for alot of them, though.
First and foremost, to become a Worgen, you need to have an account that is paid up to Cataclysm. If you did not purchase the Cataclysm expansion, then you can not be a Worgen.
Yes, to play Cataclysm you must have WotLK, like to play WotLK you need TBC.
Yes.
It is the expansion pack that comes after Wrath of the Lich King. You do not need to purchase the CD/DVD to play as you can download the client which allows you to play as you are downloading (once you download enough of it). Although you do not need a Cataclysm account to play the basic WoW, you do need it to take full advantage of all the Cataclysm features, zones, and races.
Yes, for the level 80-85 content you will need to own WOTLK however for the old world changes (changes to barrens, the cities etc) that is happening for everyone regardless of what expansion you have
Playing Cataclysm requires you to already have installed World of Warcraft, as well as the Burning Crusade and Wrath of the Lich King expansion packs.
The disk can be used to install all content up to the cataclysm installation period also buying the disk new or the install digitally online provies you with the code needed to upgrade your account to cataclysm edition. NOTE: You also need the vanilla(original), burning crusade and wrath of the litch king codes to upgrade to cataclysm and use its content
Yes. You have to have the first WoW and all the expansions leading up to Cataclysm. You don't need to buy The Burning Crusade anymore because it comes with the first WoW, but you do have to buy: Wrath of the Lich King and then you can buy World of Warcraft: Cataclysm
More than you need to learn how to be able to spell
Yes, in order to be eligible for the Closed Beta of Cataclysm, you need to have a currently active account. You also need to have your account flagged for participation, which you can do by "opting in" through your battle.net account page.