DK and Peach as characters I feel has the potential to add so much.  It's a way of introducing a new play mechanic, that makes sense.  Well, more sense then Yoshi transformations (although the car was awesome).
Nitsu: The baby floating is back in this game.  Personally, I really enjoy it.  It's a dynamic that Yoshi's Island is able to offer that I really haven't seen before.  I started playing through it again recently, and kept thinking how awesome the dynamic really is.  Most of the time in a game you are "punished" for your inability by either dying, or by the loss of a percentage of your life.  This has you physically go back and do something about it.  Big difference.  Personally it's welcomed.  Even though I don't want to imagine baby DK's cry.