By the time I finish this post, the page will probably be covered in responses, but I'll take a shot at answering your questions anyway:
These are my questions :
What's it like playing WOW ?
To be honest, the experience varies greatly depending on what you're interested in. You can quest, kill other players, gather things, explore the world or just chat with your guildies. Personally, I always enjoyed that last one, assuming you can get past the odd prick of a player.
Is it hard to get into ?
No, it's really easy. Even before Cataclysm, the game managed to ease you into the process, and although it might take a while to figure out things like threat and good spell combinations, it was still easy. With Cataclysm out now, it's actually a lot of fun as well. The quests are more interesting, mechanics are explained better, and, as always, there are loads of other people in the starting areas getting to know the game as well.
Is it as addicting as they say ?
Yes
To clarify, it's not so much a compulsion to keep doing, like I'd imagine crack might be, but you simply lose track of the time while you play. Still, I'd imagine that that could be overcome easily enough if you were determined (and maybe look at the clock occasionally).
How expensive is WOW ? Do you need all expansions ? Does the world change ?
The answer to this question is, thankfully, no. You need to install all the patches right up to the present, and that can take a few hours, once that's done, you can just hop in. If you want to keep leveling once you hit level 60, you'll need to get the first expansion, and at 70 the second, and so on and so forth. Since the release of Cataclysm, the entire pre-expansion world was redone in a much more interesting way. To be honest, the 1-60 free expansionless bits are now better than the first expansion itself, simply because of all the mechanics and interesting features that have been retroactively introduced.
As for how much it costs; 15 dollars a month is the cap, if you buy month by month, but you can get it down to somewhere around 9-10 dollars if you purchase year by year. However, I'd recommend getting a 60 day game card for $30, as it's the easiest way to sample the full game, and see if you like it.
What should I expect ?
Quite literally anything. This is the internet, so anything goes. Chuck Norris jokes and pop culture references are all the rage, so expect to see lots of people going on about those in the major cities. You might also get the occasional prick of a player, but the community itself is by and large a nice one. As for the game itself, expect lots of quests, and some grinding, though that's been balanced significantly since the expansion. Also, there are going to be a few lore references you're not going to get, and there's going to be a lot of content that'll seem remarkably empty, because everyone's now moved on to higher level stuff.
Any suggestions ? Tips & tricks for getting started ?
Gonna merge those two. Start off on a pve or normal server in your region, because the pvp (Player vs. Player) servers can be frustrating for newbies. Yahtzee would have you go horde, but the truth is that both factions are pretty evenly balanced, and pretty much equal amounts of fun. As for what class you're going to roll, there are a couple of important distinctions: Tanks, Healers and DPS. It's a tank's job to soak up damage while the DPS classes do the killing, and the healers keep everyone alive. It's not that important at lower levels, but you might want to keep it in mind when you choose a class.
Anyway, for my recommendation: By all means give it a go, but start with the free ten day trial, and if you like that then play a bit more. Some of the better features are disabled for trial characters - such as guilds - so keep that in mind when you play.
/Edit: Just found an example of one of the new, more interesting quests: http://www.tentonhammer.com/wow/guides/general/peacebloom-vs-ghouls It's basically Plant's Vs. Zombies, but in WoW.
