Principally, the type of rice used...there's many different types of risotto rice (arborio, carnaroli, vialone nano, etc) and each type is favoured by particular regions. Apart from that, there's the obvious deffierences in what ingredients are added according to regional tastes - Veneto cuisine favours the use of various spices, and seafood; Emilia Romagna is better known for its deli meats (such as prosciutto) and cheeses.