You have acquired the powers of Senjutsu, gaining the ability to use nature Chakra, as well as Senjutsu specific techniques. You have complete control over your nature Chakra, so you don't have to worry about turning into your animal type.
You have the choice of a summoning contract with either Snakes, Toads, or Slugs. Snakes are Yin aligned and have more "wizard" like powers, Toads are Yang aligned and have more physical powers, and Slugs are neutral and have healing related powers.
You start with one type, but you can take the decidedly non-canonical step of eventually learning the other two.
You normally need time to accumulate nature energy, but the time that takes varies, and there are ways to bypass this or work around the restriction.
You also become a powerful sensor type, even if you previously were not. This is to the degree that you could detect a specific person even kilometers away.
Sage Mode holds many secrets and powers, including massive improvements to any jutsu.