If grinding reputation with obscure factions wasn't enough to keep you busy, then 500 or so achievements should do it. Blizzard's put together a preview page to show off the new functionality, which will debut with the Wrath of the Lich King expansion. According to the page, achievements can be unlocked by tasks as basic as getting a haircut or, if you're a glutton for punishment, collecting 25 vanity pets.
A new character window will be implemented, allowing you to check out your own glories and those of others. The page mentions that some achievements will come with rewards, but they'll be "purely cosmetic".
Player Achievements Preview [World of Warcraft]