(I assume you're referring to the recent expansion, The Burning Crusade.)
But World of Warcraft costs typically $20 in most game stores, and the Burning Crusade expansion is $40. The monthly fee for World of Warcraft, or even if you have the Burning Crusade will always be $14.99 (I hope!).
World of Warcraft is a pretty fun MMORPG, in my opinion. I've been playing the game since it came out, and I've enjoyed most of my time playing it. I say most because, like most MMORPGs, there's always some sort of grind. Like getting to level 70, grinding faction reputation to unlock harder modes of dungeons, doing lots of quests for gold to save up for your mounts/repair bills/materials for professions etc. There's always some sort of time sink, but I guess it's a love/hate relationship with that! :]
Anyway, with the release of the Burning Crusade, it upped the level cap from 60 to 70, put in a whole new continent of sorts called Outland which contains 7 new areas complete with hundreds of quests, new factions, the introduction of flying mounts, daily quests with their own benefits, one new profession (jewelcrafting), and two new races, the Blood Elves (Horde) and the Draenei (Alliance). There's also a plethora of new armor to obtain, dungeons to explore, 25-man raids (as opposed to the old Azeroth 40-mans), beautiful areas, and epic questlines.
When will it be available? It's available now. You could just buy World of Warcraft, and if you like what you see, you could buy the expansion pack later.
Why do I like it? I enjoy the social and raiding aspects of the game the most. Getting to 70 is also a nice feeling of accomplishment, too. I like being able to get in a group with a bunch of other people and kill hard bosses and be rewarded with superior loot, then come back all over and do it again next week and hope the item I want drops. PvP (Player vs. Player) is pretty fun depending on which class you roll and where you put your talent points. And I guess my last reason out of many not listed is I like it because it's always getting updated. There's always a new patch coming out with Holiday quests (for instance, for this Halloween the Headless Horseman is a short, fun encounter that drops pretty good loot if you can't raid), world events, new dungeons, new factions, and new quests.
Hopefully my rambling shed a little light on my view of World of Warcraft. Hope this helps!
Edit: Also! A new patch is coming out in the next month or two to make leveling 20-60 easier/faster along a TON of other huge improvements (guild banks, more quests, etc.)