Quote from Mikey Monkfish :

Because they're highly tuned and run unrestrictive exhaust systems.

Myself, I'd like to see flames out of the exhaust and random pops and bangs on gear changes and when braking. Makes for more atmosphere and realism.

Mini Coopers (the new ones) actually adjust their engine timing when you're off the throttle above 2500 RPM, and the same effect is achieved. You can hear it quite clearly in this video - I blew my exhaust apart taking the fabled Dragon too hard, and the "burbles" sound like gunshots :P
http://www.facebook.com/v/163194687361

Mini Coopers also have a newer type of rev limiter, similar to what the new Fiat Punto has - it's referred to as a "soft" limiter. BMW employed this on the MINI because it has an electronic throttle, and it's much more forgiving when shifting at the limit. Example: the prequel to the video above. You'll hear the MINI hit redline several times in this video...
http://www.facebook.com/v/163005357361

Please, oh PLEASE put the Nordschliefe in! Also the Top Gear track in different configurations (Stig/SIARPC setup, open runway, etc.) would be REALLY cool.