Wow, IDK where to begin with; GT5 is by far the best racing game ever and I see very little (if not few) flaws in this game. Lemme give you, the IGN review, some pointers to this game:
On the second paragraph, you said that "Hit the anchors and it's likely to fishtail widely as it struggles to lose the kind of speed that the car was never designed for, leaving you flailing into the corner and thinking to yourself that the supercharger you just strapped on may well have been a step too far."; either you need to practice more or your skills in GT5 online are laughable.
You also said that "Customisation options, however, are limited, especially in the wake of Forza 3's more comprehensive efforts."; Excuses. That's a step in the right direction by PD, and this isn't about putting your car with countless amounts of vinyls, just so you think that you would be popular, well, it's not. GT5 is about racing with skill, precision and style, not showing off created vinyls.
Plus, you even mentioned that "when the game does show its standard class cars up close the difference can be jarring; edges are ragged and shadows sit uncomfortably, making them look awkward alongside their more polished brethren."; Again, excuses. You seem to have missed the point that Kazunori stated that the standard cars "take longer to model". You can't expect them to have interior feature on ALL +1000 cars for the next 10 years, you know. The last thing you should do is wait for future updates.
There are two things you have said (though I'm gonna make it separate):"It's fair enough that the all-new Top Gear test track doesn't tickle the eyeballs – it is, after all, drawn from a featureless aerodrome sat bang in the middle of England's featureless Surrey –"; That's like saying the SSR7 doesn't impress you, which is utterly blatant. The Top Gear Test Track IS a unique track. Don't like it? Don't race it.
"but the fictional Cape Ring displays a disappointing failure of imagination on Polyphony's part. It's a labyrinth run through a desperately bland landscape that's only salvaged by its intriguing variety of corners."; if you think Cape Ring is a dissapointing track, then you are a dissapointment to GT fans.
You also ranted about the oppoent AI becoming "now trademark zombie A.I.". Seriously WTF did you get that anyway? It's your fault you were too busy staring at the vehicle's rear bumper while not bothering to overtake. You're supposed to pass the racer BEFORE the AI makes a sudden stop; tailgating makes you look like you played too much Burnout Paradise.
Lastly, you also said that cars always bounce off even after crashing a wall at over 200 MPH. If you love to crash oh-so much, why not play Burnout Paradise or MotorStorm? Obviously, I'm not here to wreck Ferraris or Lamborghinis, I'm here to win the race with little to no damage whatsoever. With that said, GT5 is a great sim racing game; always have, always will.