>>714644535 (OP)X had better music during races.
X also had better levels overall, though there are a few standouts in GX.
Tracks are wider and controls are more simplified leading to a decrease in gameplay depth.
The rubberbanding in story missions is so obnoxiously bad that winning is less about driving well and more about pulling ahead of your opponent at the optimal moment before the finish line.
The actual speed of your vehicle has been decreased from the last game and tries to compensate for it by stretching out the camera lens above certain speeds.
Vehicle stats are meaningless and often outright lie due to the new engine.
Sega didn't understand the series art direction and everyone looks weird and kind of shitty for it.
The new characters are lame.
99% of custom vehicles are really ugly and only used to make dumb looking machines with broken stats.
The story is really dumb. I know that's sort of a given but it's kind of amazing how genuinely retarded the narrative gets by the final few chapters.