Disregard the GameSpot and other site reveiws, they know nothing about this game. It is extremely well made.
The new classes are very well made. The Dervish is a great PvE character, although not as well tuned for PvP as it may have seemed during the PvP event. The paragon has found its way into many top guild's builds for GvG and is really changing the Metagame.
The challenge missions are still there, except now the reward is gold, experience, and hero armor. The armor for characters is also very nice looking. The Primeval warrior armor is probably the best armor I have seen in all three games, and the female necromancer 5k vabbian armor is great.
The new insignas and inscriptions system is interesting, and it's really changed the economy of the game as well as the versatility of PvP weapon/offhand/armor selection.
Although people usually don't note Guild Wars for its storyline, the new writer really has done a great job with tying in the stories from Prophecies and Factions into Nightfall. Many will remember the Abaddon's Mouth mission from endgame Prophecies. Abaddon is the evil god of Nightfall, who threatens to turn the land into..well the obvious "Nightfall".
Despite the reviews of Gamespot, Gamespy, and similar websites, this game truly is one for the players. The 9.3 (right now at least) rating shows the real essence of Nightfall.