Basically Metallics are an ongoing trend that probably will last and fade by the end of this year. Whites also seem to become big, the last OMFG! Sell out bag was I'm not a plastic bag and before that the mirroir range from LV that will be relaunched soon in the Lockit model.
I would have been able to identify an 'IT' bag in past seasons, e.g. the Paddington, the Luella Giselle, whatever. But I honestly couldn't at the moment. Like others have said, there are just too many.
Big bags are still 'big', as are white and metallics. Hardware has been around quite a bit - whether it's the chunky great padlock on the paddy or the studded numbers by Marc Jacobs, Thomas Wylde or Mulberry.
To me, the overriding 'IT' bags that have been around for donkeys and are slung on the arm of every celeb are the Chloe Paddington and the Balenciaga le dix. Those two sort of stick out and seem to be IT bag classics.