I went into this having played every generation. Let's look at the cool stuff.
New type, that means you have to rethink match ups. This means dragon types have 3 weaknesses, Dragon, Ice, and Fairy.
8 way movement. No more just moving in 4 directions.
New abilities and mega evolutions. That Charizard or Mewtwo is now a Mega evolution and has another ability.
On to the bad.
The game is pretty easy and you'll get more out of the game battling with others online. Story itself, no challenge really.
The 3D. Only some 'cutscenes' and the battles are in 3D. The world map and running around, that isn't 3D.
Still, not a bad game. A good game for a long trip or something to grind it. Pokemon X & Y take the game to a new level.