Disappointing single player but excellent multi player options.
Single Player:
Events in the game take place after the original storyline and follows the familiar race-based levels minus the Orcs.
Compared to the original, the storyline is drawn out and relatively bland with repetition broken up by the occasional mini-game or event.
Multiplayer:
Online multiplayer and user created mods/maps are what make this game shine, drawing on the changed formulas and the added models it's hard to find content that isn't explicitly designed for this expansion.
While regular and ladder games do occur, these fail into a repetitive trap that eventually bores, most user created content has a way of being different enough to keep you interested.
My recommended custom game would have to be Defence of the Ancients(DOTA) Allstars.
Summary:
Don't plan on enjoying a storyline as great as the original, if your planning on playing Warcraft 3 online then this expansion is a MUST.