It's the last non-crossover one. Where instead of a bunch of flagship gundams and sazabis you have an entire roster of units featured in single universe.
Universe being Seed doesn't do it any favors, but you can pilot all sorts of grunt units and interesting models that never appear in later releases.
It's also was a transitory game between slows a fuck titles and fast as fuck titles, ending up featuring gameplay speed that actually feels like gundam rather than macross/mechwarrior.
It's great fucking game.