Concept that set Buddhism apart?

Main concept that set Buddhims apart is that of 'ahimsa' which means non-violence. This non-violence is applied to all living things and even oneself (as at the time, hindus were involved in acts of self-suffering (walking on fire, sleeping on a bed of nails)). This means non-killing for your food and at a higher level even the violence of one's words can pollute the atmosphere.

Other aspects Buddha rejected in Hinduism was the caste system, worship of idols and even the existence of God which is why Buddhism is seen as an atheistic religion. Buddha specifically asked people not to idolise him and find their own path towards nirvana (breaking of the cycle of birth and death).