The M1 Carbine is the standard by which all PDWs are measured.

[stanc]

Hind sight indicates an intermediate cartridge in a detachable box magazine would have probably been a reasonable decision.

Hindsight makes it more clear, but even back then a number of folks -- who had been paying attention to the lessons of WWI -- recognized that intermediate-power cartridges and detachable magazines was the best way to go. (See post #59 for examples.)

I think what kept the assault rifle concept from being pursued in this country is that, until the 1960s, the US Army had been very conservative, and change was done only in incremental steps.

Ammunition evolved incrementally:
- paper cartridge with .69 caliber lead round ball
- paper cartridge with .58 caliber lead Minié ball
- rimfire metallic cartridge with .58 caliber lead bullet
- centerfire metallic cartridge with .50 caliber lead bullet
- centerfire metallic cartridge with .45 caliber lead bullet
- centerfire metallic cartridge with .30 caliber FMJ bullet.

The infantry rifle evolved similarly:
- single-shot, flintlock muzzleloader
- single-shot, caplock muzzleloader
- single-shot breechloader
- 5-shot, bolt-action loaded with loose rounds
- 5-shot, bolt-action loaded with stripper clip
- 8-shot, semi-auto loaded with en bloc clip
- 20-shot, semi-auto loaded with box magazine

...comparing the M-1 Garand to a...1903 Springfield the M-1 Garand looks pretty revolutionary to me.

The M1 Garand only looks evolutionary to me. It's essentially just a gas-operated M1903 Springfield, with three more shots.