Just a continuation of Warcraft 3, and though it brings some new things to the game, all of the old problems persist.
What does this game does well?
Simply put, some of the new hero units are in all honesty, are extremely fun to use. The new Orc/Troll hero, the Night Elf Warden, and the Neutral Dark Ranger are all greatly designed. They aren't boring tank units, designed to take a million hits like the beastmaster/blademaster/cryptlord/deathknight/paladin/etc. but require lot a more finesse, much like a Lich.
Also the "extra factions" are a nice touch as well, and seeing towns and units for the Naga and Blood elves is great, but it is a shame you can't use Naga in multiplayer (yeah, they aren't as developed in terms of unit variety and abilities) but having the ability to attack from the water would add another level of depth in the game (though they would probably be gimped on waterless maps).
These are great touches to the game, but problems of Warcraft 3 are only increased.
With this game, all the side got new spell-breaker characters, which adds in more you need to do in a basic melee: Control a unit to attack another while using an ability, use support units to buff your guys and weaken theirs, use your hero (now there are even more of them) to cast spells and health self with inventory items, and now you have spell breaker units which behave like a combo of all the above (aside from the hero units). Another chore...
Also, the story isn't really improved and they just had to throw in more, poorly designed races and creeps into the mix to try to make the game world feel more "lush." The same Blizzard-Humor (read as unfunny, of the Homestarrunner caliber) is present, and it is still hard to take any event seriously as there really isn't anything interesting going on, as most of the characters are just annoying, and the voice acting is just bad.
Slyvannas is pretty cool, but Blizzard didn't go all out and just give them a separate campaign (with unique units like the Naga or Blood Elves), and yet another example of Blizzard and their unoriginality: the new faction had to be called "The Forsaken." Really?
In the end, this Expansion is okay, and a somewhat more interesting version of Warcraft 3, but if you really want a high quality RTS play Starcraft or Myth.