I wrote these two pages worth of text, trying to make a fitting introduction to Guild Wars 2 for both gamers with and without MMO experience. 
You can blame marciosmg for its existence, so if you don't like it, tell him and not me. 
Explaining Guild Wars 2
The MMORPG (Massively Multiplayer Online Role Playing Game) genre is a genre with a lot of potential, but today, many MMORPGs try to resemble the indisputably most successful MMO of all time, World of Warcraft. Many of the resemblances are seen in gameplay, which is a shame as World of Warcraft is over five years old. World of Warcraft has indirectly caused the genre to become stale and lack the evolution that is normally expected in games because a lot of developers are trying to copy ideas from a game that is over five years old and hasn't seen major changes in gameplay since its release.
One of the few developers who have been doing something about this situation is ArenaNet. They are the creators of the Guild Wars franchise, a franchise of MMORPGs where the games are different from what so many other developers try to create. The original Guild Wars games weren't fully fleshed MMORPGs, but were none the less very well received by gamers, and has made this one of the most successful franchises in the genre today.
Now, ArenaNet are upping the ante for all MMORPGs with the first full sequel to Guild Wars, Guild Wars 2. Guild Wars 2 has been in development since 2007, and ArenaNet have not wasted their time, as they are bringing evolution to MMORPGs in a big way. Guild Wars 2 is being developed from the stand point that everything should be fun, and if you don't think it's fun, you can choose to do something different, and are not locked into place by the game.
For this reason, ArenaNet are revising everything we know about MMORPGs. For starters, there is no monthly fee, because that would lead players to feel they have to play a lot to get their money's worth, and so the game locks players into place in a way. Guild Wars 2 is not a game that wants your commitment any more than you are willing to give it.
Let's go over some of the major things ArenaNet wants to change about the idea of what an MMORPG is today.
Combat:
Combat should be fun, fast and erratic, you're fighting a battle with fantastical fighters, weaponry and magic after all. In a game where elementalists can make it rain meteors, rangers can take you down from a long distance and necromancers can send their undead minions after you, players need to be nimble and not be hindered by the game. You're in a battle to the death, not a game of uno after all.
ArenaNet are making this come true with what they call “controlled chaos”. The different player professions are extremely versatile, and can change roles fast and on the fly. Everyone can revive fallen players, and when your HP reaches zero, you enter a downed state, where you still have a chance to fight your way back up, before you go down completely. There is a big emphasis on tactical movement as well, players can dodge out of the way easily, and few skills are interrupted by moving. Finally, every skill in the game is visually distinct with clearly marked effects, so players can more easily see what is happening around them.
Professions:
Guild Wars 2 has eight different professions, all of which play differently, but are very versatile as mentioned earlier. Every profession is good at everything they can do, like how warriors can wield a bow, and actually make good use of it, or how necromancers can dual wield daggers and be good with them.
This versatility is a great necessity because there is no dedicated healing profession in the game. Because of this, players have to be able to support themselves, by healing and protecting themselves and their allies. For this reason, you don't have to stand around looking for healers for your group and someone in your group doesn't have to switch to healing because you can't find a healer, that's not all that fun after all.
Story:
ArenaNet already tried to bring more actual story to MMORPGs with the original Guild Wars, but they are taking it further with Guild Wars 2. Your personal story is a fully fledged adventure with lots of choices. For starters, you have 15 different back stories to choose from when you decide what kind of character you want to play (three different back stories for each of the five races) that will affect how your story is shaped.
And how you react to NPCs will determine their attitude towards you. If you play as a person that doesn't care about who gets hurt in order to get things done, NPCs will act with fear when they see you for example.
Guild Wars 2 has dungeons as well, and each dungeon has its own unique story. The first time you enter a dungeon, you enter its story mode. When you have completed that and enter the dungeon again, you enter its exploration mode, where you get a little more story and see consequences of the things you did in story mode, but the dungeon is more open for the player.
True cooperative play:
Most MMORPGs foster competitiveness amongst its users. The person getting the killing blow on a monster will receive the reward, which can allow other players to steal your rewards. When you meet another player, you have to be in the same party to help each other out, and if you don't team up, you won't both be rewarded.
In Guild Wars 2, every player is an ally. If you are fighting a monster, anyone can come help, and you will both be rewarded fully for your efforts.
Quests/Dynamic Events:
Quests are really old school. ArenaNet wants to change how we think of quests with Guild Wars 2, and so they are replacing the quests of old with dynamic events.
In a game like World of Warcraft, you might go to an NPC with a mark over their head signaling that they have a quest for you. You receive a wall of text, explaining that a bunch of baddies are attacking a village and you need to kill 10 of them and return to the NPC with their hearts. You then go out and find these baddies standing around, waiting for you to come and fight them, you do your thing, you go back to the NPC and get another wall of text and accept your reward.
In Guild Wars 2, everything happens around you. You pass by a village and see for yourself that it is being attacked, and you then step in to prevent that attack from succeeding, or you move on and do nothing. Maybe someone is already trying to save the village, and you decide to step in and help, which is very easy since you don't have to group with these people or accept any quest in order to play a part. Then when more players join in, the event will scale to keep the difficulty appropriate.
This helps make the world more persistent than anything before it, because every event has consequences, and those consequences will play a part in a new event. If you manage to save the village, you may see an event to raid the baddies. Or if you fail to stop the baddies, you might get an event to rebuild the village. Every event has consequences, and they will remain in place until a player steps in and changes what has happened. That village will remain burned down until someone comes in to rebuild it.
And that's just some of the high points in the gameplay, the game has tons of content to boot. Hopefully we get to play this game in 2011.
