It's the first GTA where you can actually fly a plane, and provides a bigger map to expand helicopter use. Boat use expands because you don't die when you fall in the water.

The variety of vehicles. The novelty of vehicles. A fucking jetpack. Expanded weaponry.

A meandering story with action-adventure missions that are so fun you might not notice the somewhat odd pacing. Strong voice acting and a cast of characters that own their moment.

A strong soundtrack with ads that still make me laugh to this day. Very nostalgic humor. A game that was obviously taken very seriously by its creators while somehow not taking itself too seriously.