The obvious thing google flights misses over ITA is the month-long search option, which I use a lot. For the power users, ITA has a very useful regex-like syntax for specifiying flight queries, but that is understandably not of as much interest to the vast majority of people
Google Flight Search has a calendar view: it's hidden behind the button directly to the right of the return date and lets you play with different departure date and trip lengths.
