The key change is a great new start-game. The first few tech levels are now very polished, with fluid gameplay, while infantry spam is notiable only for its absense. No unit has a position of dominance, and you need everything to succeed. The towed artillery adds more units to the mix in the start game, and this added diversity greatly expands the tactics open to you.
To give a rough idea of how the early game now plays, here is a bit of information about each of the unit groups:
Basic rifle infantry (new models for the Russian soldiers btw) will now no longer use AT grenades. Instead each side has an anti-tank weapon which disables rather than destroys the enemy tank. However their main role is to kill infantry at range, and also to shoot them out of buildings.
SMG infantry are the new addition, and are more geared towards close-ranged combat. They mow down rifle infantry at close range, and also can use smoke grenades. They have HE grenades which will clear out a building faster than riflemen can do it, but only from a closer range.
Light tanks now have a more clearly defined role in the early game as a heavy machinegun platform. Infantry will have a hard time taking one out, as they are only really vulnerable to the dedicated AT infantry. They are exceptionally nasty units to large groups of intanry, and provided they have soldiers to take out any AT men, they can clear a street very fast.
Mobile Anti Air plays a big role in the early game because of its power vs infantry (the Russians now use the M17 which is basically a lend-lease M16). While it is no match for light tanks, it can massacre infatnry in the open. This helps somewhat to ease the game into the more tank-oreintated later phases. Then again, if you neglect infantry you will lose... It's a simple as that.
Light Artillery is the offensive support weapon. It can lay down withering fire to massed formations, and also suppress enemies or conceal your own forces by laying down a smokescreen.
Light AT Guns are deadly ambush units, and can be used to make strong defensive positions. They will shut down a blind rush of light tanks with no difficulty, but against artillery they are exceptionally vulnerable.
You may notice the increase in power a lot of units have vs their targets. Rifle inf can clear buildings with their standard attack, and SMG infantry are even more effective at this at close range. Light tanks can take appart large groups of infantry, and mobile anti-air units even more so. This means that the gameplay is fluid. A garrison can be cleared without too much effort, and massed formations in the open have plenty of counters. After all, if someone is really determined to dig in, then you can always break out the light artillery and blow them to bits!