The "Double" part, the introduction of Toadette, Wario Colosseum, DK Mountain, Waluigi Stadium, Daisy Cruiser, Baby Park (with double items bringing chaos), special items, Bob-omb Blast, unique karts, unlockables everywhere, and my brothers/cousin liked to play the game as well so I got full multiplayer experience whenever I wanted.
*Note, technically Double Dash is my 2nd favorite Mario Kart, but it was my first video game that I loved instead of just enjoyed, so it's very special in my heart. :P