I have Green version. And I'm not lying, I'll even take a picture if you guys doubt me. Here's what I have to say:
*It's pretty much EXACTLY the same as the US Pokemon Blue
*In the US Red/Blue, you know how the tileset in Route 23 looks a little different than the rest of Kanto (And it looks like G/S/C's Kanto tileset)? Yeah, well, in the Japanese Red/Green, that tileset is used for the entire Kanto region. Basically, the main tiles are different.
*Sprites are different, of course
*Battle menu when selecting attacks is a lot shorter, it's more of a square shape rather than rectangular
*For some reason you can put 30 Pokemon in each box. This is also true for the Japanese G/S/C.
*Intro displays Gengar and Nidorino, unlike the US Blue version which displays Gengar and Jigglypuff
*Red's sprite on the title screen is different from the US Red/Blue
*The layout for Mewtwo's cave is COMPLETELY different. It's a lot more similar to the FR/LG layout.
Also, apparently the JAPANESE Pokemon Blue had adjusted catch rates and improved sound, according to Bulbapedia, and since the US Red/Blue is a modified Japanese Blue version, they would have these "upgrades". Sound-wise, I don't hear any difference between the Japanese Red/Green and the US Red/Blue.