Past a certain threshold (I'd say about tank tech 4), infantry armies can't do much offensively anymore and they take a backseat to the tank and bombers. With the Americans I had planned to max my air and use infantry and rely on UK tanks, that was a mistake. Basically you need to have numerical advantage in tanks as much as you need numerical advantage in air.
Sugar said before that this game is really one of attacks and counter attacks. It is very true. There are very few areas where a committed defense can be (somewhat viable). If your opponent attacks, you better give way. In NA and Southern Russia my two biggest disaster on the field happened when I stuck around too long when I should have withdrawn my units.
I think we saw an example on how rail allows you to skimp on garrisons. While Sugar's France was guarded by a single unit in Paris, I had corps in NA protecting my rear that would have been massively more useful bolstering the frontline (and protecting my air!).
I thus think that an argument and a means exist to remedy each of the above:
1. Allowing Infantry Tech to either go above 2, or give them anti-tank capability. Remove motorization as an applicable tech to infantry (give the fully motorized US/UK armies an extra move point, only allow the Axis/Russians to have their dedicated motorized inf. units, which was historical), allow A-T up to a certain level. Hubert added those 2 attacks to tanks early on in SC2, but with no countering boost to infantry.
2. Already discussed implementing limits on operational movement. Since there is already precedent in limiting amphibious units, would seem like a natural to me.