Couple of the most notable things from this stand alone expac:
You will be able to take your heroes with you into not just the PvE game, but there will be specialized arena's to fight in vs other players who have their own chars and heroes as well... So you could have all 4 chars controlled by you VS someone else with 4 chars...Heroes
NightfallTM brings more than a dozen Heroes to your aid as you battle against the forces of darkness. Each of these Heroes will gain experience at your side. You can even equip them with new weapons, upgrade their armor, and choose their skills for them. Best of all, you will be able to direct your Heroes in battle, telling them when, where, and whether to fight at all. You'll even be able to activate their skills for them. The amount of control is up to you. Use them like enhanced henchmen or direct them much like extra, full-fledged characters. The choice is yours.
Skillz skillz and more skillz!!!!!New Skills
Nightfall brings 350 new player skills to the game, including brand new skills for the six core professions (Warrior, Monk, Ranger, Elementalist, Mesmer, and Necromancer) and the Guild Wars FactionsTM professions (Assassin and Ritualist). Additionally, Nightfall debuts the brand new Skill and Attributes panel, which will be your one-stop menu for customizing your character and Heroes for the battlefield. A new template system will allow you to save your own custom builds for every contingency.
I like the idea of custom templates as well, as I have to switch all the dang time