The Japanese version of Might & Magic has graphics for an owl plaque in the Sorpigal Gallery. That definitely doesn't exist in the American version.

The Japanese version has quite a few differences in the Sorpigal Gallery, although I'll wait to play the ROM and get some actual screenshots instead of posting the tile viewer images.

Anyway, the Owl plaque I mentioned seems to have been replaced with the Corak plaque in the American version. The plaque that actually shows the robed figure of Corak is not in the Japanese version's graphics.

The plaque image for the "One by Land, One by Air, One by Sea, and One by Sand" does not exist in the Japanese version.

The plaque with the Dragon head on it does not exist in the Japanese version either. 

The plaque with the Treasure Chest also does not exist in the Japanese version.

The Japanese version, however, has a plaque depicting a large hourglass that doesn't exist in the American version.

Plaques that are the SAME in both versions:

The Castle Doom plaque

The Chessboard plaque

The Wheel of Luck plaque

The Gala plaque (which looks strangely similar to the opening scene from A Link to the Past that shows the sages sealing the Golden Land.)

Again, I'll get images for comparison later, but I'd rather present actual screenshots for these.

Oh, one more utterly bizarre and inexplicable change - All the enemy designs in both versions are identical - sans two. The Snake/Slither Beast enemy and the Vampire Bat enemy in the Japanese version are both only represented by one creature. For some inscrutable reason, these enemies where changed in the American version so that they're represented in clusters of three (see this site again for reference). Since these are common Sorpigal enemies I'll try to screenshots of them soon rather than post the jumbled tile viewer images...