I've been playing this for quite a bit now. What I really like is how everyone in a party gets the same amount of experience that they would if they were fighting a monster alone. To the best of my knowledge, most MMOs divide and distribute experience between the party, which to me, discourages teamwork and social interaction. As you can imagine, these should be things that people should value in multi-player games.
In Realm of the Mad God, nearby people automatically become a party, without the need to send or accept any invitations. This makes the parties incredibly spontaneous and the teamwork very uncoordinated, but still a hell of a lot of fun.
Once when I was playing, I entered a dungeon with at least 30 people. With so many people around it became incredibly difficult to coordinate attacks and as a result, people were dieing left, right and centre. When our numbers were reduced to around 5, as a team, we could coordinate attacks incredibly easily, and avoid further casualties.
I really love it how you can have a lot of fun regardless of the size of your army.