Some features that will come with Nightfall:
# New continent Elona with 4 regions (Istan - Egypt style, Kourna - Savannahs and mud huts, Vabbi - Arabic themed, Desolation - poison desert).
# Two new profs (Dervish, Paragon).
# 300 new skills (for Factions classes also) + 50 copies
# 4 new guild halls.
# 250 Quests, 55 explorable areas, 20 new Story Missions, and three new Challenge Missions.
# Hero NPCs that you can control, move around, set style (aggressive-defensive-passive) and customize (armor, skills, weapons and secondary professions). You can even control what skills they use.
# Hero vs. Hero (3+1 vs. 3+1). Each player controls 4 (!!) characters. RTS style?
# Skill templates save option (finally!).
# Salvageable item inscriptions (bonus modificators).
# Cut scene NPCs now move lips.
# Shader and bump mapping engine upgrades.
# Emphasis on story telling and PvE. Nightfall will tie the first two chapter stories together and it's been said deliver a "Apocalypse" that will also be seen in Prophecies and Factions.
# ...
Check your
http://guildwars.com/support/gameupdates/ for more info about changes and features.