RISE FROM YOUR GRAVE!
Yeah I found this game fairly recently but didn't think to look for a thread about it here until, well, right now.
The biggest thing I've noticed so far is that turning Supply on or off has a much bigger impact on how hard the game is than any of the other difficulty settings. The need to stop and get more ammo and/or fuel every few turns can really slow a major advance down and turn fighting an enemy position into a protracted slugging match. Air power in particular suffers badly, since (as I found out the hard way the first time I played with Supply turned on) any plane that runs out of fuel before it gets back to the nearest airbase is toast. And it all tends to handicap the AI less than it handicaps the player, since in most levels it's your job to advance and their job to try and stay put.
On the up side it's adding a whole new level of tactical depth that I didn't really dig before. Infantry are no longer rendered useless, since the need for everything else to occasionally stop and refuel means that even guys without motorised transport are able to keep up with the front line. Vehicles that I used to dismiss as suboptimal suddenly look like hella smart purchases since their increased fuel/ammo capacity lets them keep on doing their thing longer than their top of the line counterparts. And capturing airfields is a major priority since having airfields close to the front has a drastic affect on how useful your planes are.
Also, the Kaiser General and KUK General campaigns for World War One are both total ass. Everything except artillery is much better on defense than on attack. Artillery (even with transport) takes forever to move to the front and set up. Blitzkrieg advances are out because cavalry is made out of tissue paper and tanks are nonexistent until the late game. Air power is pathetic and really only useful for scouting and artillery spotting. And while this is all nice and realistic, it makes for an incredibly dull game because PGForever's game mechanics really aren't good at making a slow-moving protracted infantry slugfest fun to play.