Commercially, Freespace 2 was considered a failure - and thereby killed the entire genre of 3D space shooters. No major publisher has dared to release one since.
Many people believe, however, that this failure had a lot more to do with the utter lack of promotion given to it by the publisher, rather than lack of market or quality. Quality wise, it was definitely better than the original.
The expansion campaign for the original, however, definitely fits the "failure of quality" description. Silent Threat was utter tripe in terms of storyline and gameplay, although the other elements were at least serviceable.
However, after porting the original game and the expansion to the open-sourced FS2 engine, the community rewrote Silent Threat extensively, and came up with considerably improved story and gameplay - to the point where it is as much worth playing as any of the better user-made expansions.
They even managed to retcon it into better continuity with FS2, eg. the Hades being destroyed in Deneb (as seen clearly in the FS2 intro cinematic) rather than elsewhere (as in Silent Threat original). Admittedly that was due to the cutscene artists not paying attention, but who can blame them?
The key to knowledge is not to rely on people to teach you it.