Destructive environments (trees, shrubs buildings), vehicles, sound effects can NOT be beat, game-play is more tactical.. but still fun with arcade, bullets drop from gravity at a distance, spot players, get rewarded for more acts than just killing, true squad play, larger maps, more war like feel on assault and defend game modes, better graphics, able to use surroundings as camouflage...
Vietnam just plain OWNS ... and I never felt that about a Vietnam game before!
Just a few of what I personally think. I am sure others can fill in with more detail.