According to the terms of use, yes
Ownership/Selling of the Account or Virtual Items.
Blizzard does not recognize the transfer of WoW Accounts or Blizzard Accounts (each an "Account"). You may not purchase, sell, gift or trade any Account, or offer to purchase, sell, gift or trade any Account, and any such attempt shall be null and void. Blizzard owns, has licensed, or otherwise has rights to all of the content that appears in the Game. You agree that you have no right or title in or to any such content, including without limitation the virtual goods or currency appearing or originating in the Game, or any other attributes associated with the Account or stored on the Service. Blizzard does not recognize any purported transfers of virtual property executed outside of the Game, or the purported sale, gift or trade in the "real world" of anything that appears or originates in the Game. Accordingly, you may not sell in-game items or currency for "real" money, or exchange those items or currency for value outside of the Game.
This question gets a bit tricky to answer... Starting from scratch, if you buy just vanilla WoW, you can experience the original world as it has been destroyed by the Cataclysm expansion. You can level up to level 60, but cannot access professions, areas, races, or other such things from any of the other expansions.
In order to access Outland, jewelcrafting profession, and Draenei? / Blood Elf races and starting areas, and level 61-70, you will need vanilla WoW + the Burning Crusade expansion.
In order to access Northrend, Inscription profession, and Death Knight class, and level 71-80, you will need vanilla Wow, Burning Crusade expansion, and Wrath of the Lich King expansion. Oh, and a prior level 55+ character on your account to play as a Death Knight.
In order to access the misc Cataclysm level 80+ areas, Archeology secondary profession, Worgen / Goblin races, and level 81-85, you will need vanilla WoW, Burning Crusade expanion, Wrath of the Lich King expansion, and the Cataclysm expansion.
Those are the restrictions. You can play on whatever realm you want with whoever you wish. You just may not be able to reach all the areas they can.
You can trial every bit of it with the 10 day trial, though I don't quite know how they treat it when your time is up.
Best Answer
Great question :)
I wondered it myself and googled it before trying, so here is a good discussion on the issue;
http://tobolds.blogspot.com/2007/01/how-big-is-azeroth.html
And a mentionable and scientific! approach in a comment :)
Mages can use "blink" to jump forward 20 yards. So, I blinked forward five times, ran back and measured the distance a few times. Averaging it out, you run 100 yards in 14.15 seconds. 100yards / 14.15 seconds = 7.067 yards/sec Three yards to a foot, means 21.201 feet/sec. Which works out to 14.46 MPH. Meaning, in 18 minutes 35 second you actually ran 4.48 Miles. So, lastly, 4.48 miles / 3.7 * (6.4 miles on each side [The square root of 41 square miles]) = 7.75 Miles on one side. 7.75² = Kalimdor is closer to 60.1 Square Miles
For ref: Manhattan is 20 square miles
read all the comments, they made quite a discussion on the issue..