New IPs in the MMO sector have risen and crashed with tidal regularity for the last decade thanks to WoW, but Guild Wars was one of the few that didn't get the rejection slip from the mass market.
Its developers originated from Blizzard's work mines, but left to form ArenaNet and create an instance-based virtual world largely in response to the MMO tropes that became standardised by WoW.
You might remember the game's business strategy, for one – an anti-subscription based model that solidified ArenaNet's role as one of the few high-end MMO developers willing to go the free-to-play route. The game also opted for an instanced world instead of a persistent one. This meant your experience in the game was relatively personalised, and that you weren't sharing the environment with hundreds of others at a time.
The sequel similarly tries to forge new paths, bur really this is an entirely different beast altogether.
The recent trend for MMORPGs leans toward the story-centric RPG side of the equation, resulting in the development of both this and Star Wars: The Old Republic. Guild Wars has gone back to using a traditional persistent world as its base, but rather than running the risk of seeming derivative - as Old Republic is accused of being in practice – this has helped it become one of the only MMOs in development with a clear sense of identity, post-WoW.
The quest system has been overhauled. Beyond the fact that they've done away with the exclamation-marks-atop-of-quest-givers routine, Guild Wars takes a page from Warhammer Online and throws in a dynamic event system - to put it in human English, large-scale, random events that anyone out and about in the world can join. Players can meander between set quests they've picked up, and Events like the taking-down of giant Dune-styled worms or the wiping out of pirates raiding a nearby town.