Excellent. Blizzard continues to have great games, not to mention its first-class map editor and customization.
Campaign: Warcraft 3 had an excellent plot, and the expansion also has a good plot. Undeniably intricate and intertwined, the plot maker might have been chosen to write a play that would have been locally acclaimed. If the mainstream campaign wasn't good enough, the bonus campaign, focusing on the RPG element of the game, adds another flavor to the game. Cinematics add to the game as it portrays scenes that are often appreciated by campaign fans (I do not belong to that sector). But it has not been flawless, with the blasted cliffhanger that is the problem with the EA's C&C games, combined with underdeveloped plot involving the Forsaken (which was pretty cool, but then no new units and underdevelopment was a sore disappointment). It creates a large gap in the plots, such as the fate of the Blood elves are never explained, nor is the fate of the other humans ever been covered in depth.
Gameplay elements: Blizzard's unequal but balanced factions fare very well. The new units are often anti-magic, allowing for nullification of magician threat. Naval units are added, although it is underdeveloped and are very rare. Perhaps two of the greatest additions revolve around heroes: Player-owned vendors and mercenary heroes. Taverns can instantly awaken heroes (with penalty) and allow for recruitment of merc heroes that are not native to any faction and are specialized in a particular realm, such as bolstering units with summons, reducing enemy effectiveness, or brute combat. Gameplay is also sped up with lower cost and increased hit points, which emphasizes training yet maintains the importance of troop preservation.
Customization: Starcraft had awesome map editor, but Warcraft III has an even grater map editor. With the expansion, custom scripts are possible on the normal editor, and one can do almost anything. Even better, The expansion comes with an ability editor, a big plus to customization. One can make RPGs with proper models that are easily imported and implemented (other than cumbersome pathing). Blizzard's tradition of customization is perhaps its biggest advantage besides its excellent gameplay.