OK I have looked into a few of these issues in a bit more detail and come up with the following.
Track Length
Every source I have looked at for the real Nurburgring lists the Nordschleife's length as 20.832 kms, which converts to 12.94 miles.
The official GT4 guide lists the games track length as 12.94 miles, which does match. However being the untrusting individual that I am, I took my brand new (0.00 miles) VW Golf GTI '76 for a gentle trip around the Nordschleife, aiming for a 12 minute lap (for reasons I will get to).
I paused the lap the very moment that the lap counter clicked over to 2 (the car was doing less than walking pace at this point) and the on-screen odometer was reading 12 miles, however this only reads to whole numbers.
So I quit out of the lap and headed home for a quick look at the garage mileage reading (which is to 1 decimal place), the result 12.9 miles.
I have to say thats accurate enough for me.
Track Width
This one is still a subjective opinion, but personally having visited the Nordschleife on a number of occations I do not believe that the width is that far out. It must also be taken into account that the game does not view the world in exactly the same way we do, and depending on if you use an in-car view or external will change the perspective.
The two shots above contrast the real with GT4, both are taken around the Brunnchen section of the track, and while its in hardly scientific, again I would say that the differences are small.
Brightness of the track
I must confess that this one annoys me more than anything else, Germany is allowed to have good weather, with nice bright sunny days, as the real shot above shows.
If you prefer the Enthusia look and want the track to look darker, just pause the game and use the display option, set the brightness a bit lower and away you go.
Bumps
Now I am not silly and to dispute that GT4 has exagerated a few of the jumps would be just that, silly. They are a bit over the top, but to be fair so is the way that we take them, I've always back off from them on my visits and I'm damn sure that no competitor in the 24hrs would want to ruin there race just to get a bit of air.
Would you do this in a Primera, I would in GT4, but I didn't in reality!
However the strength of the bumps in the GT4 version I do not feel is over the top (but Enthusia's version is far, far too smooth). This was the reason I aimed for a 12 min lap, the time Wolfe2x7 says he go around in, it makes a massive difference to how they feel. 12 min laps are an average of 60-65mph, compare that with an average of 90mph for an 8 min lap.
Additionally the road car bias of the rental BMW suspension would not be similar to the set-ups we use for most of our cars in GT4. Try a gentle 60mph average drive aroundthe GT4 'ring, you may be surprised at the difference it makes.
Start/Finish line
Famine hit this one on the head when he quite correctly stated that GT4 uses the original Start/Finish line for the track. Now while the official site shows the new Start/Finish on its map (as does the site for the 24hrs), just about every other map or site you look at (including a BMW track guide I have, uses the old Start/Finish line.
Guess this one must just be down to history.
Overall
In my personal opinion PD did a damn good job with the GT4 version of the 'ring, it still gets my vote as one of the most accurate around and the following post which contains extracts from an article with Nick Heidfeld regarding driving a new M5 on the 'ring and how it compares with GT4.
Nick Heidfeld & the 'ring
I hope that the above is of interest and helps to explain my take on this subject.
Regards
Scaff