The features in FireRed and Leafgreen were really neat. First, I liked how the game showed a "past preview" of what you did when you last played in gray-scale. It gives a nice reminder of what I did and I can refresh my memory of my last gameplay.
I liked seeing the item icons when I selected one and I had a nice visual of how it looked. And when an item was used on a Pokemon, and it showed, that was neat too.
I also liked how there were image previews of some places like caves and forests. That was another good visual.
I could mention more, but I guess everything else in the game fascinated me.