Forged Alliance does offer more, but introduces new problems. It's still good, but maybe not better than SupCom.
Forged Alliance expands on the success of the original SupCom in the area of audio and graphics. It's technically more of the same, but on a bigger (Personally, I would say EPIC) scale.
The campaign of Forged Alliance will better illustrate just what I mean by saying epic. Of course, there are other RTS'es that require more of your concentration and time. But what Forged Alliance does that nothing else can is the tactical view. With a simple downward scroll of the mouse, you can see everything, and I really mean everything, that's going on in your map. I won't spoil the storyline for you, but you will honestly feel that the campaign might just be a tad imbalanced.
In terms of multiplayer and skirmishes, I would say nothing much has changed. Except that you can only use the Seraphim if you have Forged Alliance, that is. While this limitation is understandable, it certainly doesn't make it any more appealing to new players.
So what's lacking in this game? Quite simply, I would say that the larger scale and more detailed textures are a doubled-edged sword. Even with a 2.4Ghz Intel Core2 and a 8800GTS, my framerate stuttered occasionally during the campaign. It was expected that my PC would choke a little when things got too tight, but I shudder to think what would happen on lesser machines. I play SupCom and Forged Alliance with everything set to max.
So there you have it. Forged Alliance isn't bad by any means, but when put beside the original SupCom, it just doesn't look as awesome. Fans of the old SupCom will have mixed feelings about the new expansion - especially the new UI that I'm still trying to get used to. However, I would recommend that prospective gamers looking into the SupCom series should start with the original.
In short, Forged Alliance isn't bad, but it's not as good as SupCom