From: South Carolina, USA
The growth rates seem very different from v.2. Infrastructure improvements are much slower, more expensive, and there are more ways to go about it. The economies are reluctant to switch over to rearmament/war production, and even the 5% boost to PP can result in a single PP added to monthly production. This five percent boost costs the United States 166 PP on a base production of 64 PP. We are not going to see massive economies supporting hi-tech armies in the early war. You will be fighting with what you brought, for a long time.
There are restrictions on supply storage. They may seem severe to those used to a more liberal policy. Certainly we will not see the British GHQ holding 24 months of secure supply. Warehouse wastage will destroy supplies if boundaries are exceeded. Shipping available to the militaries is far lower than we are used to seeing. Shipping a single division will strain your starting resources.
The SFTs have been extensively modified. Purchase strategies from v.2 may not be optimal in v.3. Bombur has also added lots of nifty new units, especially to fill out the "fight with what you brought" scenario. The French mention above that they have a paucity of their best armor. They have 4 units of B1-bis available, and they are expensive, and seem not as strong.
Diplomatic areas often replace individual countries. You influence Norway and Denmark together, not each one. The Allies may well want Norway, but defending Denmark will be tough.
The demands on a nation's resources are greater than ever. Should you support forces in Spain? Should you aid the Chinese?
The v.2 player will have an advantage over a newb I think, but he must not let preconceptions blunt his play.