Leaps and bounds ahead of the original Supreme Commander.
Forged Alliance pretty much beats the original Supreme Commander in every way possible. First of all, pretty much every single unit in the game (excluding walls) was rebalanced. The balance of the original Supreme Commander was primarily based around the "Mass Fabricator," which was able to convert energy into mass. You could pretty much only build up a powerful economy in your base for the first 15 minutes of the game, then try to get a quick experimental unit out before your opponent. It pretty much broke down into whoever can build their economy faster. Definitely not too exciting.
Forged Alliance totally flips Supreme Commander's balance on its head. First of all, the T1 mass fabricator is moved to T2 and it uses -150 energy to make 1 mass; it's practically useless. Since you can't get mass by just sitting in your base, the game revolves around fighting over control of the map and being able to build mass extractors on the deposits around the map. The more of the map you hold, the more mass extractors you can build, which means that you'll be able to build more units to defeat your enemy. Commander upgrades are much more useful, air is seen much more often, factional diversity is improved, and the game is much more faster paced and exciting.
Forged Alliance is generally much more polished than the original Supreme Commander; the graphics are better, the UI looks much better, range rings are incorporated, the Fog of War is much smoother, etc.
If you bought Forged Alliance just for the single player, then you're really wasting your time when you could be having so much more fun online. The SupCom community is pretty mature, especially on the official forums.
If you have any questions, you can send me a private message on the official forums (http://forums.gaspowered.com); I post under the name "Ertwyu."