This expansion is brilliant.
There are new units, storylines, maps, and cutscenes that take you deeper into the Warcraft universe.
The bonus orc campaign is incredibly fun; I've played it several times and still get as much entertainment out of it (Blizzard wanted to create some kind of orc campaign, but didn't know how to introduce them into the expansion; and they also wanted to make some kind of movement to world of warcraft, so they put two and two together to create a type of role playing campaign where you control only the heroes).
The alliance campaign isn't actually focused on the humans, but the blood elves; who have a slightly darker side revealed to them.
The undead campaign is split between Arthas, champion of the scourge, trying to protect the lich king; and Sylvanas Windrunner, the high elf general (slain by Arthas) who has gained some of her own control due to the waning powers of the lich king, driven to build her own army of free willed undead (like her), to end the reign of the evil lich king.
The one dissapointment I had with this game was that you have to start playing as the night elves to reach the further campaigns; I just despise playing as night elves, I find them too weak and just not as good or fun to play as the other races. I really hate how high and mighty the night elves act over the other races, despite being the most difficult for me to play as (due to the fact that to take out a party of two orc grunts, I needed to rally around seven of the night elf archers).