It is vastly different in many ways, but the things that spring to mind first and foremost are the improved defenders (if you have been bothered to adjust them in the formations screen), the lack of cutback goal opportunities, the keepers fantastic saves from shots outside the box, the fact that you can switch the play from wing to wing (if you have a man in space and if you can master the manual long pass), the AI of the rest of your team when attacking i.e they make some fantastic runs and move into space.
I also love the fact that if you can't seem to penetrate your opponents defence you almost always have to 'start again' and pass the ball back to your midfielders/defenders to try and build up the play again.
Also, the so called superplayers i.e Adriano, Zlatan, Ronaldinho and the rest are not quite as 'superior' as they were in PES6. Adriano could score from any angle and from standing still at any time he wished in PES6. Not so anymore.