Just steer clear of WoW lol, if you get hooked expect your life to change violently for a long time to come, I just got out of the game after a 3 year stint a while back and it was the best thing I could've done. When they talk of wow bieng as addictive as cigarettes, alchohol and crack they aren't joking.. your relationships with friends and family will deteriorate, your sleep will suffer and your work performance will go down lol. If you join a raiding guild you might as well just cut your losses and put a catheder and a bucket next to your pc. I made the mistake of introducing 4 people I know to wow, and I created 4 monsters who don't socialize with the outside world anymore and spend every waking moment on that game. I've asked them to quit or reduce thier playing time and to go out and get some sun for a change, but they remain as pasty and malnourished as always
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Of course if your willing to risk it and think you won't become an addict then no you don't need the expansion right away but for the price of the game+ the expansion you might aswell indulge. If all else fails most game stores offer a 14 day trial of the game for a paltry $1.99 so you can see if you like it and want to continue. Be aware tho, as a noob to the Wowverse you can expect to be mocked and made fun of if you don't have a group of friends to hold your hand for the first while.
Edit: I'm sure I might get flamed for my opinions, but while an estimated 25-30% of the wow population are true casuals and can take or leave the game as they please 70-75% of people are not and get consumed. Most of these people are folks with addictive personalities and people that want an "escape" from life and themselves. For me I was in the addictive personality group and only my declining interest in gaming as a whole and my newfound love for the 360 saved me from my WoW demons! lol, My name was Dandraseth and I was a WoW Aholic!.