Soosiz, a gravity-bending platformer that played well on the iPhone, looks killer on the iPad.
As with many of the other games on this list, the added screen real estate with properly scaled controls means that your fingers don't get in the way as much. However, the scenery itself takes up about the same amount of size, meaning you're not going to be able to see more of a level than you would before.
The only odd thing about the HD rendition is the pair of black lines on the top and bottom of the screen. These give the game a movie-like appearance, but in truth, it would be nice to have the game take up the full resolution of the iPad instead.