The AGA and ECS or OCS chipsets are very different.
Teh AGA chipset has a greater palette and can allocate more colors on screen at once than previous chipsets, and it has lots more display modes too..
So the main difference betwen an AGA game and an ECS/OCS game, is that you will, at least, see more colors.
Also teh blitter speed is different, so some stuff goes faster. And hardware sprites can be bigger.
And since A1200s come with 2MB Chip de-facto, games can have more graphic and sound information.
Hope that helps!