Im going to keep this strictly pve based, even though Im in love with GW2 pvp. PvE, for me, is quite an experience in this game compared to the standard MMO.  Keep in mind this has no personal story comments and is after the very first boss event. My story:

In the GW2 beta you probably started off with a huge mob of people running to your first event. As expected you blast through it and the mob moves on. With each event the mob starts fragmenting and splitting into different directions and you keep moving along to whatever draws your attention the most, looking at the map every once in a while.

You see this guy's farm under attack from bandits from a nearby cave and decide to help out. Unfortunately there aren't many people left from that big mob and you have to run and escape. Inside you feel a bit sorry for the guy but you know your last fight can't be for the sake of a farm, so you run...

You end up in a nearby town, looking and listening to everything around you and do some training with the local army. All of a sudden you hear a plea of help. What do you know! Its the farmer and all of his cattle in the middle of the small town square! Now you think, you know what! Im going to help this guy and make up for my lack of true "heroism".

So you run back to the farm with a handfull of others that are also answering the call for help and have a fierce battle to drive the bandits back into their caves. While pushing them back you see others fighting some huge ass Drake, and decide to join in the fray. After all this, you notice you gained a level in the process and some loot. 

Now, compare this to your standard MMO...you start off talking to someone and he gives you a quest/task, you do it,  you check your level bar to see how close you are to level,  you talk to someone else and get a quest/task, you do it,  you check your level bar to see how close you are to level,  you talk to someone else and get a quest/task, you do it,  you check your level bar to see how close you are to level. I think you get the picture......

For me in other MMO's its just a grind to see my level bar go up, Im not really interested in just exploring because my level bar is priority #1 and exploring is not efficient. In GW2 I found that I didn't really care for my level bar and my priority #1 was just to explore, help others and just soak it all.

Everytime I logged out I had no clue what level I was and just thought about getting back in to check out that cave I saw in that place. This to me is what GW2 is all about, its a completely different and refreshing experience. Not a level bar grind...