The added feature of Heroes (characters that grow and travel with the player) revolutionized this whole series.
A Hero is a partially playable character. The user can control the Hero's actions to an extent, including what armor they wear, how it is upgraded, what skills, spells or signets they carry, etc. Pretty much anything a user can control on their own character, they can control on a Hero. A Hero, however, can't complete quests or earn titles, except those that help advance the story of the user's character. Heroes may be directed individually, or with the rest of the party as a group by "flagging" them to locations.
One of the drawbacks of many role playing games is the AI "help" that is given to the user. Usually, that help is great in some instances, and sorely lacking in others. With a Hero, that help can be configured on an as-needed basis to serve ideally for the task at hand. This is something I'd never seen done quite so well in a game before.
I have to add that the artistic dimensions of this new continent are quite impressive. The use of Egyptian and African motifs and imagery is quite brilliant, and adds a great dimension to the world that ArenaNet has created.
The humor cranks up a notch from the previous two GuildWars games on this one. One of your Heroes is a warrior named Koss who has a giant afro and white armor. One of his quests has you fetching him a pep tonic to make him strong, and...well...I won't spoil the jokes.