From what I've heard (granted, my sources are limited two three pro!3rd season people), it's mostly that "the good guys are bad and the bad guys are good and they are such lovely shades of grey and the characters are now so multidimensional and human." Because I'm still going "lalala, my Heroes ends with the season 1 finale, leave me alone, lala", I have no idea if it's accurate or not, obviously, and... no desire to find out, tbh.