What's it like playing WOW ? In all honesty, it's a grind. While I quit before Cataclysm came out, a fair share of, and the entirety surrounding endgame is about doing the same thing repeatedly.
Is it hard to get into ? No. In fact, the learning curve on it is whittled down to almost nothing, especially in the beginning. Learning to be good at your role takes work and patience, but picking the game up and playing is extremely easy.
Is it as addicting as they say? That depends on you. Are you the type of person who always needs to get further, get that next level, that next piece of gear, or see that new zone you haven't been to? If you are, then you may find yourself staying up late on the weekends, but it's all about your level of maturity and ability to pull yourself away and get back to the real world.
How expensive is WOW? Wow is $15 a month. A reasonable cost, especially if you do get into it and its the only thing you do. As a former military man, it's certainly cheaper then going out to the bar or going clubbing every weekend. Admittedly, if your on a tight budget, MMO's in general and especially WoW offer a lot of bang for the buck.
Do you need all expansions? If you want to get to max level and see all the content. Yes. In fact, your going to need all of them if you want to see the updated starting areas that everyone is playing in now.
Does the world change? If by, when you kill a boss does it stay dead? No. For the most part, anything that you do will be undone after a cooldown timer when the thing respawns for the next person. Again though, having not played Cataclysm, I can't comment on what changes have been made recently since I know they are using phasing technology far more.
What should I expect? This depends on you and whether you enjoy the game or not. I can say that no matter what server you go to, expect to meet the gamut of people. From folks who you will form long time bonds to, to people who you would like to push into a fire.
Any suggestions? Don't play it. While Warcraft may be the big kid on the block, it's certainly not the best, nor the most recent. If you want to get into MMO's find one that appeals to what you enjoy. Aion, Rift, admittedly, playing anything other then WoW will help further the MMO genre by taking a body away from Blizzards monolith and putting it somewhere else. Of course there are other MMO's coming out too that you may enjoy. Tera and Star Wars: The Old Republic come to mind, as well as Guild Wars 2.
If you want to play WoW though, my only suggestion is to not tie yourself to it. I understand that after spending countless hours working on a character, and making some friends along the way, it's hard to turn around, leave it all and start over. It's like moving to a new city, away from all your friends...but sometimes moving is the best way for us to grow, and if you find you enjoy it, tell your others friends to come over and play it with you. In the end, it's a game, and a game should never make you feel like you have to continue playing it if you don't enjoy it.