Great intangibles like heart and determination. Fitness. If you watch him against Patterson, etc., you will see that he had a lot of ability that he doesn't get credit for. Since he was never knocked down, people think of him often as just a punching bag, which is completely untrue.
Chuvalo always came forward, never stopped, and had better speed, punch combinations and power than many realize. Just watch his fight with Doug Jones, a guy who gave Ali a tough time. You'll see. Chuvalo IMO was the first guy to lay out the blueprint to beat Ali. He was the first guy to consistently work Ali back to the ropes and force him to grab. Frazier took that game plan to the next level with even more consistency, offensive output, head movement, and crisper punches.
He had pretty good power, his defense certainly wasn't great, but he kept his chin tucked, and he had plenty of stamina and was relentless in the ring.
Do you agree with my observation then? He comes across as sombody with an encyclopedic knowledge of boxing technique but not the talent to implement it to its full potential.