IanL already listed some of them: reaction times, vehicle situational awareness, and survivability. A couple others are weapon system accuracy and reach. I'll explain the issues below.
Thank you Ian. I think I've been treating tanks as their modern counterparts (especially US since I usually play them in CMBS); I have implicitly assumed a relatively high level of independence in spotting and survivability that simply doesn't exist in the WW2 context. And in the modern era, I don't usually move infantry through the open except if they're to go somewhere stealthily because of the lethal airbursts (BMP-3s and arty), so thanks for your advice to lead tanks with infantry.
Another issue I've been facing is that the infantry fights seem quite different. I've noticed several things:
1. Infantry die much more easily to small arms and grenades. Modern infantry are more resilient to these. So I don't know how aggressive I can be with them.
2, There are plenty of explosives in the modern era and I've always relied on them (one of my favourite things about the US is their grenade launchers) and suddenly there are no explosives here and no "I will use this ATGM to kill that strongpoint". People make up for that with arty, but I don't know exactly when and where to call arty especially given the long response times and lack of precision and airburst arty.
3. The Soviets suffer from a lack of infantry firepower at range and I can't really move thinly armoured stuff like SUs forward too early, which makes me clueless about assaulting defensive lines (I'm not very well-versed in the use of massed artillery so tips are welcome here).