Warcraft III was simply fantastic, there were few ways a game like that could be improved (most of those ways had already been taken care of with frequent game patching to stop cheese strategies), but somehow, the Frozen Throne looks like a complete new game, not just an improvement of Warcraft III and so, it's better than the last one.
The new campaigns are awesome, there are a few missions you will repeat many times as they are so fun to play, I rememeber a mission where you have to get a cage to your base (je je, spoiler free!) and have to fight with it, sort of a golems game from battle.net, I repeated that mission about 15 times before I won in the first try, and every time it was really fun. The story is a bit strange, but I think it's quite good, and the videos are amazing, just as the first ones, you have to look at them, so realistic, even Final Fantasy looks up to it.
But there are two campaigns, the "normal" campaign which narrates the race to the frozen throne and the indcredibly cool, overwhelmingly addictive campaign of the orcish horde, where you controll a Beast Master and a Shadow Hunter in some missions in Durotar and it's outskirts. This campaign plays like a RPG with the hero system, makes you wonder why weren't all campaigns like that. An excellent bonus for all of us. Besides it includes the Demo campaign, which is called the same as the introductory campaign in Reign of Chaos but actually it's different, and it's quite good too.
Gameplay has been improved with some excellent ideas, for example, if you press the Alt key you will see the energy bars for all the units, if you press the [ key you will see the energy bars for your units and if you press ] you'll see the energy bars for all the enemy units, an excellent addition, as it helps throwing spells and curing your units. Other changes include queuing units and research and queuing buildings, everything adds up to a much better and easier to control experience.
I think sound was great in Warcraft III, and I have to say that music is one of those things that weren't improved, but music isn't that important and music was already great so there was nothing to improve. But voice acting is as good as ever, and voices have always been one of the best things in the craft series.
The game is indcredibly fun, and you'll find yourself playing against the computer for hours, because the computer changes strategies (and I'm not sure, but I think it also chooses strategies to counter yours, because I keep loosing to the computer by using a strategy that defeated it easily the last time), and if the campaigns and the games against the computer weren't enough, you can play over the internet with the greatly improved battle.net, overall the game is excellent, if you haven't bought Warcraft III, hurry up and buy it, just so that you can play the expansion pack.