Maturity. Blue belts learn techniques. Purple belts develop a coherent game. The difference is between knowing 50 moves and choosing the 5 that work for your body.
Technical depth. A purple belt has answers — they understand position, transition, and counter at a level that lets them control the pace of any roll with a lower belt.
Teaching ability. Most professors expect purple belts to help white and blue belts. If you can't explain a technique clearly, you're not ready.