The difference of an IMAX bluray and the regular Bluray is during the aspect ratio changes in the theatre, you get the use of your full screen vs having black bars at the top and bottom
here for example
the top picture is the scene on the regular blu-ray and the bottom picture is the scene from the imax blu-ray (although this particular pic is talking about the colour differences from pirated releases)
oh here another pic showing the same (without the colour issues)
mind you only the scenes in Imax Aspect Ratios will be full screen and those that aren't will have black bars too