TL;DR, GW2 is obviously an MMO, you play with tons of people online, but in my opinion it is really more of a single-player experience. Fespite the fact that you enjoy it with other people it's very personalized to you and what you want to do, rather than you conforming to it and everyone around you. It simply has the added benefit of being able to quest and do dungeons with other people.
It has tons of people in an online persistant world, so obviously it is an MMO right?
Ehh... Maybe.
I hear all of these comparisons to other MMOs, and quite frankly none of them are valid. Guild Wars 2 is not a true MMO.
What is the point of an MMO? Or rather, what do most people THINK is the point of an MMO? Simply put, to consume one's life and give them something to do forever and ever and ever regardless of how repetetive it is, just so they can tell their friends how awesome they are at gathering 1337 pixels. Right? That's what modern MMO's do, they give you a world to live in, literally. The concept of 'fun' is almost lost in the grind.
Can Guild Wars 2 be considered new world in which we can spend every waking moment? Probably not. It's quite honestly not designed to keep people playing for eternity. Why? Obviously because of the lack of subscription (inb4 people saying this is a terrible defense to bring up). Rather, it is designed to pack an immense amount of content into a single-player style MMO. It is designed to give each and every person an experience they will never forget. It is deisnged for you to actually have fun while playing it. Sadly, no one sees it this way. They want the ability to rise above the game, to get that loot at the end of the tunnel. They want that dungeon that they can run a hundred times just so they can get the right combination of gear, just in time for another repetetive dungeon to be released so they can do it all over again. The problem with that kind of game is the experience is completely funnelled into one action. Guild Wars 2 breaks that mold.
Guild Wars 2 is not at all traditional. It basically throws all the concepts of an MMO out the window. The game doesn't start at max level, it starts before you even create your character. It doesn't force you to do anything, you can play it any way you want. You can even craft to max level. If you want to keep playing after you've done all the dungeons and personal story, you have a TON to focus on. Above all else, it isn't about gear, it isn't about loot, it isn't about shiny things, it is about enjoying the entire experience. Simple, honest fun. However, if you want shiny things, you can still get shiny things, but only by actually playing the game, rather than adhering to a strict sect of elitist raiders that most people can never hope to measure up to. It doesn't adhere to the elite, it adheres to everyone. And for some reason, that makes the elite feel left out.
But what's one of the biggest components of it's anti-MMO-ness? Scaling.
Regardless of how many people are around, the events scale. You can have 100 people doing an event, or you can do it by yourself. The amount of people is basically irrelevent except in group events (which can be done with as little as two people, and solo'd if you're at all decent), and therefore it plays a lot more like a single player game. The only difference is there are actually other people doing the same thing. If you want you can come back to GW2 long after everyone has moved on and play it on your own as a single player game, with the exception of dungeons of course.
So, does it play like an MMO? Not at all. It plays more like Diablo 2 only with more people in the open world. It's a multiplayer singleplayer game. You do most of your work alone, but alongside other people. You can do your quests and events by yourself, there just happen to be a ton of other people doing them too. Am I making sense? Probably not, but oh well.
TL;DR, Guild Wars 2 is a Single Player game that you happen to play with a lot of people, and eventually you will beat the game. That being said, it is the single most valuable, innovative, interactive, and outright beautifully polished single player game in the history of the internet. Enjoy it, because it is glorious.
/rant
EDIT: This was just posted in another thread, it's from the original lead WoW developer:
The feeling of grind melts away, and you just enjoy yourself. That’s the key, because all long-term (100+ plus) online games are about progression. The grind is still there, as it has to be for any progression based game. But the manner in which we go through the progression of Guild Wars 2 is fresh and unique enough to keep us interested and entertained for hundreds of hours. They did to the WoW formula what WoW did to the EQ formula. You see, quest hubs and such were heavily promoted by Blizzard to hide the fact that you were just grinding mobs to level (which was the original EQ formula). You were, in fact, still doing the same things as EQ, it just felt like you had a purpose and a story that guided you through it. It distracted you from the grinding. I remember early EQ players in beta for World of Warcraft. They would just run out into the fields and start killing monsters…and be disappointed. When I asked them if they tried the quests, they would always react with shock…they never tried them, or in some cases, even noticed them. But once people started questing, it was a completely different experience, and the rest was history.