Of all the real-time strategy games I have played, I have never come upon a game that has frustrated or satisfied me as much as this one. Though it is no surprise that Blizzard could stir those feelings of resentment and satisfaction, World of Warcraft did the same to me though I did kick that habit.
The main storyline of Warcraft III is superb, I for one, love the lore of the warcraft universe and playing this game before i played WoW made the experience all the better (even though blizzard massacred it in WoW). The campaigns are superbly crafted to provide a challenging and fun experience, even if the unit structure is a bit weak but that is overcome by a great storyline and memorable characters, i know I'll never forget Illidan and Malfurion Stormrage, or Grom Hellscream. The cut scenes that occur after every campaign are amazing as well, but blizzard is known for their prowess in that field. Just look at diablo 2's cut scenes between each act.
The battle.net custom games allow for many different variations of the game as well, most notably the Tower Defenses and Defense of the Ancients, both of which were spawned from Starcraft versions of themselves.