My Battle.net account only contains World of Warcraft and The Burning Crusade. However, I've noticed that I can create a Worgen character, even without the later expansions installed. Why is this?
With the release of 5.0.4, all races except Pandaren are now available to all no matter what expansions they have on their account.
Once Mists of Pandaria is released, you will also be able to make Pandaren.
Races are no longer locked to expansions. You can download the free-to-play version of WoW and create a character of any race available in the game.
Classes which were introduced in expansions are still locked to those expansions.
You must have Wrath of the Lich King to make a Death Knight (as well as meeting the other requirements).
You must have Mists of Pandaria to make a Monk.