I'm and old TA player and found a lot of trouble getting my rythm on Forged Alliance. Here, based on my experience. I want to make a quick-start guide in order to help short this learning curve. Feel free to help.
0 - Now we have 3 full techs, not 2, and also a 4th experimental tech, like the Krogoth.
1 - DON'T THINK LIKE THIS: "Ah, Supreme Commander puts the 'S' in the RTS, before, it was tactics, no more APM intense games as the deciding factor." Actually, this is 10 times worse in the beguinning. The games normally last half the lenght of TA, mostly because of the stupid fast start due to small unit that made HUGE differences: The engineers. All factories make engineers now, instead of their respective builder. And they are cheap, and made very fast by the factory. You don't need to have a commander assisting your vehicle factory just to pump up the start expansion, actually, you will have a headache managing all the engineers at the start, while doing all the other stuff.
2 - The commander
Now he has upgrades to research and don't come with a huge D-Gun. On the upside? He have a lot more life. So you can actually use it, (and have to), on the battlefield. He only starts doing 100 damage per second, and the basic upgrade makes that for 200 dps, but thast as much as it will go. Besides Overcharge, that its only good for units that are close togheter basically. It takes a long time to recharge and the comm is clumsy and takes long time to aim when is surronded by a lot of small units. Some upgrades make him almost a superhero - epic build power, epic shield or a big laser gun for cybran (and teleport), but they are as expensive as they are good.
3 - Balancing support
Total Annihilation had tons of units, but on hardcore play, we only used a handfull, because they weren't balanced. This is not the case now. Eventhough there aren't as many t2 land units as we had Kbots, they all serve for a purpose and have a decent cost/benefit ratio. Some weren't as good on the game launch, but thanks to this great comunity, that's being fixed.
4 - Factory and engineer build speed
The t2 and t3 factories now produce in a decent rate and don't need much assistance from engineers to make lot's of units. Biggest exception would be t2 and t3 ships. T2 and T3 engineers are also way more balanced to his role - a T3 engy can actually make a T3 power plant in a decent time.
5 - Extractor and Factory upgrade themselves.
One of the biggest changes: The mass extractor now upgrades itself, and the high tech actually is worth making, instead of doing the Fusion+MohoMetalMaker combo (Thanks to the proper balacing). So you won't spend time reclaiming a t1 extractor, just to make a expensive thing that don't give much back after all. The same also aplies to the factories, they also upgrade themselves. Those two are great for relieving some unwanted APM.
6 - The scenarios are much more reclaim worthy
Tons of trees and what not, you really HAVE to make engineers just to reclaim the place around. Most maps this helps a ton. ALWAYS look around the maps to see if there are rocks and some wrecks to get for a mass boost. Thankfully, engineers on patrol work much better than that clumsy T1 constructor vehicle.
7 - Tactical Missile Launchers
8 - Shields
9 - Some units now work as intended
The nuke really looks like a nuke now, but damn, it costs a ton. The same works for the battleship, specially the UEF, now with a more realistic range. And the t3 or experimental artylherie, they have a huge rance.
10 - Some units are now useless
Remember the Guardian - that epic t1 arty, that had a decent range and very good fire rate. Well, there's no T1 arty turret, and the T2 does a good amount of damage, but fires only once every 20 seconds You probably won't build any of those in 90% of your games. T1 subs also do very little damage now and probably some more that I'm forgeting.
11 - Energy is totally rebalanced
You will find yourself very frustraded that the T1 Power generator takes a ton of time to be build and also costs a lot for how much it provides. It still gives you 20 energy units, but that is worth much less now. Factories, Power generators itself, commander upgrades and mass extractor upgrades all costs a ton of energy. What about planes? They still do the same amount for t1, but t2 is way more expensive and t3 is ridiculously expensive. T4 experimental units are also a big power hugger. You will spend a long time relearning how much energy do you need to make whatever you are planning.
12 - Strategic bomber
This is a new t3 unit that's really expensive but can cause havoc on the enemy base.
More to add:
When the "S" in RTS is actually aplied
Aero transport and ferry
Proximity bonus
4 races and each main difference.
???
So far for now, I only had in mind writing about #1 lol.
This same exercise should be done for those who are coming from Sup Comm 2 and another for a general RTS player maybe.