I don't think you can generalise that all "geniuses" are in some way mentally different from us.
Einstein is famous for a number of reasons but probably has no more right to the title than somebody like Bohr or many other less well known scientists.
However there is such a thing as Savant sydrome, usually autistic sufferers who can do stunning mathematical tricks, tell you which day of the week any date is and things like that.
see
http://en.wikipedia.org/wiki/Autistic_savant
Daniel Tammant is an interesting example though because unusually he can communicate and function fairly normally living an independant life and has recently published an autobiography - he famously learnt Icelandic in a week for a television program.
http://www.optimnem.co.uk/
On the other hand Kim Peek
http://en.wikipedia.org/wiki/Kim_Peek has severe disabilities but has some ofthe most startling abilities able to read two books at a time for example with incredible recall (he was the inspiration behind the Rain Man film).
Unlike Dainel Tammant and most autistic savants he has structural brain irregularities.
So I'd say that most people regarded as geniuses are not that different from you or I but there is a sub group - the savants who are.