Vicious hand to hand combat, every inch of the way to Tokyo. Every child, every woman would become suspect, and horrific civilian casualties would mount. It would make some interesting moments at the Nuremburg War Crimes trials, with Nazi defendants pointing to the massacres in Japan and claiming they weren't any worse...Napalming villages and whole cities likely wouldn't have made the newsreels back home.
The Russians would have invaded the North islands, and it would have been as bloody there as well. Then we'd have faced a dividied Japan, much like Germany and Korea. Imagine fanatical Japanese Stalinists....yikes!
Just because we can easily figure out how to protect armoured vehicles from insurgents today, doesn't mean that it would have been easy to do then. Sure, the guys were just as smart, but getting material into Japan to fabricate the mesh and shields..that's harder. There's also the lag time between being attacked by kids with sticky bombs and women with satchel charges and figuring out how to prevent those attacks. The easier, on site method would be to shoot anything that moved. Then you get logistics kicking in...how much ammo can be shipped into Japan how fast? How long before orders come down to preserve ammo?
Japan had an active biowarfare unit, so I think biological/chemical weapons would have been fielded by them. All the old WW 1 poison gasses, and some new, not so nice stuff cooked up too. And even if they didn't have an atomic bomb, they might have been able to field an radiological bomb, a "dirty" bomb. It wouldn't have killed anyone immediately, but after a while...Laying in wait, until large units had moved into an area, and poof! A whole army of fleet dusted with lingering death.
Sadly, the ability to nuke two cities in Japan is actually the better outcome. What a sad world we live in.
Alvis 3.1