I've been playing WoW since it came out - and I can offer a few things.
1. Friends play it. It is VERY easy to get new people into the game, get gear, pvp, etc.
2. Achievements - like xbox gamerscore, when these were added, it pretty much doubled the game - sure they are useless in a mechanical sense, but they do offer very cool Mounts and Titles
3. PvE - Dungeons (or Instances) are the main focus, and they are VERY well designed. The variety of boss fights in the game now are simply amazing. And, if at 1st look the graphics may look cartoony - check out video fights of Algalon, Sarth+3D, or anything from Sunwell (just google, or search youtube for those topics). Competition among end game guilds is fierce, with rankings adjusted daily - World and Realm firsts for a variety of things earn incredibly bragging rights.
4. PvP - The flip side to dungeons - Battlegrounds (CTF, CC, and 'kill the general' and Arenas (2v2, 3v3, and 5v5) are the other half of the game. Every class can add something to any fight. Leveling on a PVP server is a must, world conflicts can flare up anywhere - even in low level zones where higher characters will try to make life tough for people leveling.
5. BIG ONE: SUPPORT! The WoW forums are active and the moderators/developers/tech assist are extremely helpful and post a ton. On top of that, sites like www.wowhead.com and www.mmo-champion.com can aid players with a database of everything in the game and the most current news about the game. Lastly, podcasts such as "The Instance" (available on iTunes) are funny, informative, and interesting.
6. Graphics. Many will point this out as a flaw - but WoW will run on almost any machine with any connection. Sure, intense PvP and hardcore raiding will require solid machines and a cable connection - its not really needed to still experience the game.
7. Something for everyone: Like I mentioned above, there are Dungeons (which come in 5, 10, and 25 player modes) and Battlegrounds. But what if you don't like either? Then leveling different characters can be enjoyable - I've leveled 12 characters past 70 - and each time I go through the game, I travel to different zones and experience different things. Even trying Horde, then Alliance - both have different areas of the world to explore and quest in.
8. Professions: WoW has the most basic, yet wonderfully detailes profession system of any MMO I've played (almost all of them, except Aion atm). Gathering - where you find minerals, herbs, skins, etc - can be used to feed into a Crafting Profession, or sold to other players for HUGE sums of gold. Which you can buy just about anything with.
9. Economy/Auction System - if anyone is familiar with FFXI, there is one major problem - the money never goes anywhere. In WoW, a system of AH fees, mailing fees, and repair costs (as well as huge money sinks of super expensive mounts) keep the gold in the game from crushing everything else. But the best way of doing this the concept of "Soul Bound" items. These items, once put on a character, can never be traded (there are a few exceptions, recently added).
I could go on on, but I hope you get the picture. Download the Trial - but more importantly, find a friend who plays - they can recruit you to level you faster, and get a unique mount. And leveling with a friend is always easier - in any MMO.
The last thing I would like ot point out, is that I could also make a list of things wrong with WoW - no game is perfect. But WoW is the most balanced (in terms of what you can do, not specific classes - thats a whole other forum) and most fun to play of any MMO out there.
I hope you enjoy!
Its easy to play and theres plenty to do for everyone
