As you may remember, the original plan for the massively multiplayer role-player Guild Wars was that rather than charging a subscription fee, they'd release full-priced expansions every six months for people who wanted further adventures. When Guild Wars Factions took a full year to appear, that plan was presumed to be abandoned. With Nightfall arriving almost six months to the day later, it seemingly hasn't.
While the original Guild Wars was based in a traditional fantasy world, and Factions took on an Eastern flavour, Nightfall heads into Africa and the Middle East, with settings that seem one part Prince of Persia, one part Tarzan and one part Indiana Jones.
It's also a lot less linear than Factions, leaving room to explore. Most excitingly, it moves closer to a traditional RPG in its addition of sidekick heroes to the game. Rather than the faceless Henchmen, these sub-heroes are gained along the way and develop directly in power with you. You're able to bring any three you have with you on a mission, and they can be directly controlled like pets.
Equally, they have their own skills, which can be reconfigured according to your preferences, so letting you work out devastatingly effective power combinations for your group. Since different heroes have different personalities, some will refuse to associate with one another.
Like Baldur's Gate, this means that your associates will become pleasingly differentiated from other people's as you play, depending on whether you sided with the wizard-hating fighters, or the fighter-hating wizards.