There's been no difference since the begin at all. Yeah tiny bits and upgraded graphics.
Sorry >_< I hate soccer/football
Uhh...
Now, I'm not an avid FIFA fan but even I see a huge difference between FIFA games. Not on a per-year basis (that's usually roster and graphics upgrades) but whenever they jump engines or introduce some new innovation, you can "feel" it. You're only saying that because you haven't played many FIFA games or you haven't played them a lot.
Most of the differences lay in the camera panning, tricks, ball control and goal keeping - they don't strike you in the face because it's all football, but you can feel them when you actually play the game and try to do some nifty moves.