1. Tsunade and healing. Tsunade is extremely innovative within the art of healing. Unlike many ninja, she is second to none to Hashirama, and the best medical ninja alive. Apart from this, she was able to save people from her village, and is able to save allies around her almost immediately. Even her enemy, Orochimaru, wanted her to heal him.
2. Iron fisted will. Tsunade is someone who will stand strong with her opinion. She really has an iron leadership and iron emotional qualities. Someone like Kakashi is way too relaxed all around, and seems to only be serious within combat situations. Apart from that, he tends to waste chakra way too fast within combat, this cripples him as that "iron fisted" candidate. Tsunade can think tactically and isn't like her fellow sannin.
3. Being a Sannin. Unlike everyone else who was slain around her. Her and her two allies were able to survive Hanzo. Hanzo was notably one of the best ninja of his time, and someone who was extremely strategical.
4. Care for her village. Like many others, she shares so much care for her village. Being a descendant of Hashirama Senju, she has such great passion for her village, and will sacrifice her dignity to save it. Of course this applies to everyone, but Tsunade has shown this on multiple occasions.
So what if she's only a healer? Every Hokage specialized in certain fields.
Hashirama in Mokuton.
Tobirama in Suiton.
Sarutobi in general arts.
Minato in speed and spirit.
Tsunade in healing and will.