It's not just ISKCON. We have many schools, and regional differences. There is no 'mainstream' just a conglomerate of many schools. One could just as easily ask how Hinduism in Guyana is different than Hinduism of Sri Vaishnavism in Andhra Pradesh.What is the difference between the ISKCON Hare Krishnas and 'mainstream' Hinduism? The only surface difference that I see is that they believe Krishna is the source of all deities and incarnations whereas most other Hindus think Parabrahman isthe supreme God and is formless. But Vaishnavas, Shaivaites and Shaktiites think Vishnu, Shiva or Durga are the supreme. respectively.
Another thing I don't like about Hare Krishnas is the Ratha Yatra where they push and shove each other out of the way for fruit prasad. I personally found it embarassing.
What else makes ISKCON differ from Mainstream Hinduism?