Not especially stellar necessarily, but most definitely the best MMORPG you can buy right now.
The world of Warcraft (no pun intended) truly comes to life with the stylized visuals which fit with the Warcraft 3 theme. They truly do have a unique sense of style and have developed an interesting game world with plenty to see and do. The graphics and effects are probably some of the best that I at least have seen in an MMORPG.
As for the gameplay, their motto must have been KISS (Keep It Simple, Stupid). You can easily dive into the game and figure out what you are doing without ever looking at the manual. In fact, I honestly don't think I've read a single word out of the manual to this date lol, and I've had this game for a long while now. But don't let the simple interface fool you. This game has plenty of depth, especially once you get into the higher levels and get to dabble in your talent trees. That's when the real fun comes into play.
The game consists of two factions; the Alliance, and the Horde. If you have ever played a Warcraft game (and if you haven't you must live in a cave), then these are instantly recognizable. Beyond the factions you have 4 races per faction with classes that possess unique abilities. Each class plays totally different, so if you ever get bored, you can always roll up a new class and try it out.
There are a million WoW servers. You have Normal, PvP, RP, and RP-PvP. Which goes to show you how dedicated Blizzard is to this game and the community. They have consistently been rolling out good patches and adding even more and more content without the need of an expansion pack (although the first exp. pack, Burning Crusade, is scheduled for release sometime within the next century).
With the good gameplay, stylized visuals, plenty of quests and "instances" (i.e. special "dungeon" areas), a simple interface, and character customization through the Talent Trees, and you have yourself one heck of a game. There are secondary "trade skills" you can pick up and take part in if you don't feel like doing quests.
That's another thing about this game. You can easily level fast by doing quests, so the days of hacking the same monsters over and over are gone. There are more quests than I can even begin to count, and that's only per faction. Taking in the game as a whole and you have a massive collection of quests and things to go do.
As for the loot, there are some very cool items that you can get. The auction house is a great addition, something that EVERY MMORPG should have (although Blizzard was not the first to have this idea, but it is implemented really well).
One minor quibble I have is that it would have been nice to have longer names or names with spaces at least, and a little more customization in the character creation department, but this doesn't really affect the game as a whole.
All in all, if you are looking to play a MMORPG, at least right now, World of Warcraft is easily hands down the best choice. Now, we shall see once D&D Online comes out who is the reigning champ :)
Now, WoW doesn't exactly usher in the next generation of MMORPG's, but with what's available on the market, it is easy to recommend.