An objective look at this game.
What the game IS:
It is, without a doubt, the most stable, content filled MMO ever made, period. It is for these two reasons, mainly, that it is also the most successful game in the history of mankind. Basically the game is split in two, between the warring factions: Alliance and Horde. You start by choosing one of these, and then a Race and Class. Afterwards you are provided with a fairly lengthy level "grind" (Especially if you've never done it before, and and extremely so if you have little experience with MMO's).
However, the bulk of the game's content is geared towards the end game, that is once you've stopped levelling. Once you have arrived at this point, the two avenues for advancement comes down to PVE and PVP (Plaver vs Environment/Enemy, and Player vs Player). Both of which are stacked in tiers of advancement, and are dependent upon building your way through these tiers. I.e. to do the more advanced PVE or PVP you will need acceptable gear from the previous tier of these in order to guarantee a reasonable chance of success in the harder avenues.
Whichever way you choose to go, the game provides an almost endless amount of playtime in either direction. You would be hard pressed to find someone in either PVP or PVE that can go no further in their gear acquisition. PVP gear is less chance based to acquire (that is, once you've earned the necessary items to acquire it, you can simply buy it, rather than having to wait and wish for a particular boss to drop it), but is subsequently harder to obtain as other players will be directly competing against you for the ability to attain said gear.
The game and its developers also provide a steady stream of NEW content, even aside from the major expansion packs that have been/will be released for it.
The game provides a large amount of character customization. The actual appearance of your character is fairly customizable, and the look of your armor will be fairly unique from others. The main strength of the game's class system comes in the customization of talents for each class, in addition to the already present difference of gameplay mechanics for each class. For most classes, the talent trees allow basically three different types of subclasses for each class, with understandably allowable meshes of these subclasses. They can always be respecced to a player's choosing, so nothing is ever final in the game.
A good hobby. Whether you enjoy dungeons or fighting other players, there's plenty to be done here. Also, the various world events/holidays in the game are agreeably festive, and fun to participate in. Also not completely without their rewards.
What the game ISN'T:
Innovative would be the main thing here. YES the game has tons of content. YES the game is stable. NO the game does not provide ANYTHING that you can't see elsewhere. Be it the classes, the way the world is built, the crafting system (Especially the crafting system), nothing is ever innovative or original. How much this fact detracts from the games value for you is up to you.
A changing world. NOTHING ever changes in this world. Some may find a large measure of comfort from that, while others will not. Cities do not get invaded and taken over, Terrain does not change, items do not change, crafting does not change or have variation, materials do not change. Static would be the best word to describe it.
An immersive game. This is arguable, I agree. However, for those of you that have played this game, ask yourself: Do you REALLY feel like part of the Horde or Alliance armies? Are you ever called upon to defend or attack things? I would say: No. The prearranged "battlegrounds" for PVP are instanced off, to be contained in their own little worlds, where the outcome of battles cannot effect the players around them. Even if a major Horde/Alliance leader is taken down, nothing really changes. And the opposition that killed them weren't doing it for glory or honor, they were doing it for the achievement and the rewards that ensue.
A consistently balanced game. Blizzard has always had the notoriety of making EXTREMELY well made games. However, it has also, in my mind, had the notoriety of making extreme, game changing patches. It was true in Starcraft, in Warcraft III, in Diablo II, and now in WoW. Unfortunately, I think there are clearly some classes that have been favored over the years. The real knock on the head when it comes to these patches is the severity to which they can change the game - completely breaking a class, and perhaps making a demi-god out of others. This mainly affects the PVP side of things, but in the end, it can leak into all facets of gameplay.
A game with a good crafting system. Yes, there are THOUSANDS of recipes, and perhaps a few hundred materials to be used while making them. However... there is, in actuality, VERY few reasons to bother. As I said earlier, the game is geared VERY heavily towards end-game content. Nothing wrong with that, but every time you increase the level cap of players (It was originaly lvl 60, then 70, and now 80, soon to be 85), it completely obsoletes EVERY crafted items before that. So at any given time, fewer than 5% of ALL crafted items across ALL professions are actually useful to the bulk of the players. It would also be fair to say that even the bulk of these items are ultimately useless, as their non-crafted PVP and PVE counterparts are far and away better.
Conclusion:
If you've never played an MMO before, and would like to try one, I would definitely say start here. It is stable, not hard on your system, doesn't require a thousand dollar upgrade to play, and for $13 bucks a month, is hard to beat as far as a reliable timesink. The learning curve is not incredibly steep until you hit the highest possible level (lvl 80 at the moment); after which, in order to get into any worthwhile dungeon or PVP group, you will need devastatingly good gear, as well as expected to be a master of your class. However, once you've experienced the bulk of what the game is about, I will say you will find that you'll be looking into a neverending stream of repetition and lack of change that goes hand in hand with the nearly endless content. There are tons of dungeons to be had, as well as a good number of PVP opportunities, but once you've experienced them a couple times, that will be that. And unfortunately, a couple times through anything is hardly ever enough to get what you want, so a couple turns into a few dozen. Some don't mind this at all, but I find the game eventually digresses into a series of daily/weekly chores that must be completed in order to insure your success in the game. Good luck with it, and I hope this was helpful.