In my review of the Japanese art vs the US, I have found a few differences, based on my limited review. 

First off, Optimus Prime.  Not only does he go by a different name in Japan, but the box art is totally different.  Looks like similar pose, different artwork.  See for your self below.

Next on the list is Shockwave.  I believe in the US quest to have all the boexs matching (window on the left, and art on the right) they flipped or altered picts so that the robot was facing in towards the left.  Shockwave here was flipped like this, and its a flaw since his gun arm should be his left, reverse the image and its his right.  The Japanese art is correct having him facing the right.  Same image, just flipped.  It appears to be inspired by the original Pre-TF art, but the original looks better to me. 

Soundwave, the one that started my investigation.  In an earlier posting about a Fake Microman Cassette Man, the seller was educated (as were we all) when picts of the Japanese and real MM version were displayed.  The art on the original Pre-TF box is almost identical to the Japanese box (cept of course Cassette Man is covered over).  The US version has him holding his gun down at his side, not down and out as the other versions do. 

Finally we have Megatron.  The Original discovery was in how he holds his gun, down in the Pre-TF version, and straight out in the TF.  I did not see any change to the US version with the Japanese, however I did take another look at the version of Megatron released in Japan with out the scope or stock pieces, as well as the original Black Microchange version.  The art used in the Japan version is based on the original MM art, however due to the color differences it appears they had to redo the entire thing. 

Other than the fact that the Japanese Mail order boxed toys have sketch art, while the US versions had plain brown or white boxes, I did not see other variants...yet.

Special thanks to Tripredacus, Himawari
and AFA G1'S for your help. 

Not only was the Japanese SOundwave art different, but the toy was a different mould as well. The silver buttons and tape door hinges were really different.

All the best
Maz

And as for artwork, other obvious standout differences are the WHite Astrotrain, and the exclusive camera-mode artwork used on '21' Reflector for the Japanese Takara TF release.

Also the "Reflector" is misspellt as "Reflectr" on the boxart

All the best
Maz

I didn't include Astrotrain in the search because there was no Pre TF version to compare with, though as a G1 that was later remade black and white (I know I have a boxed original one...slightly yellowed) that would be an interesting side by side look.

I thought about doing reflector, I am sure MICROX is probably a lot different than Reflector...did not know it was spelled wrong. 

There is also the 3 robot art from somewhere, the mail order catalog?