I have been playing WoW since first release. I started on the Dalaran server the day of release and the the server went down for a couple of days but I stuck with it and have been there ever since. I play Alliance and have a level 80 of every class except Warrior. I think I will wait until Cataclysm is out and then level the Warrior as a Worgen.
I am playing the Cataclysm beta and think once again taht the game is being remade (the game no longer resembles the original except for the scenery. With the next expansion even the scenery will change.
I suggest to the original poster to pick one character and level it - as stated before Hunters and Warlocks are great to level because you get a built in tank to walk around with you (your pet) while you dish out massive damage. But go visit all of the current starting zones and see other bits of scenery before it all gets remade. After level 20 and until level 60 levelling really speeds up. Plus, levelling is easier now because you get mounts at lvl 20 and epic mounts at lvl 40. Buy new equipment form the AH as soon as you can afford it. Join a levelling guild who will join up with you to do quests. Also, use the dungeon finder to do all of the level appropriate dungeons. As a DPS it takes you a while to get into them, but your role is pretty easy and you will be less likely to tick off people who have been playing the game for 4 years and are on their 7th alt when you don't know Shadow Fang keep like the back of your hand.
Take care to create a well balanced character especially if you intend to keep playing it into the expansion. When I say well balanced I mean don't forget about secondary professions like cooking and fishing. Fishing is a pain to level if you do not do it as you go along. End-game rewards for fishing are REALLY worth it.
Also, make use of wowwiki and other such sites that will give you hints.
One last thing - don't buy gold. You can earn enough gold by picking up skinning and either herbalism or mining. Sell everything on the auction house. That's easy money and the lower level mats actually sell at crazy prices - on Dalaran briarthorn sells for 20g a stack.
I could talk about the game forever, as others have said for $15/month I get the most bang for my buck of any entertainment opportunity. I can play by myself or with friends, and it has never become a grind for me because I can always find something new to do in the game.