The reason the cars can't overtake right now:
1 - Over-Reliance on downforce. Spa had tonnes of overtakes even without the DRS because the wings were set so small and more of the cornering relied on mechanical grip.
That is true (didn't consider that); getting rid of diffusers should help the situation, limit the wing angle allowed, make the cars slippery, less turbulence off the rear...
2 - The awesome braking systems. As was mentioned by DC and Brundle in commentary the brakes on the modern F1 cars are just awesome, they are starting their braking much later, which means less of a chance for a bonzai up the inside.
Indeed, I have been saying that brake strength should be limited; making braking distances longer creating more opportunity to be brave on the brakes to overtake...
3 - The anemic engines. The V8s really don't have the low-end speed of the older engines.
Yes they have less torque at lower revs but it's the same for everyone, so it really makes no difference, maybe less wheelspin...
4 - Runoff areas that are tarmac instead of gravel traps. This gives drivers the confidence to just run balls out the whole time. If they run wide then they have no problems rejoining. if it was gravel and it would end your day or make you take a long time rejoining it was more of a risk to go balls out. This is a double edged sword, as it also helps the overtaker... But I think it helps the overtake more since they can defend harder.
I despise the tarmac run offs; I believe that a driver should be punished for going off track, sometimes more time can be gained by going off track, these are supposed to be the best drivers in the world; they should be able to avoid the run off areas, right now you can go off track and lose no time whatsoever...
5 - A lot of the tracks just aren't suited to overtaking. Some are too narrow and some have the idea that you need a bunch of short runs followed by corners. As we see in Monza and Spa, Japan and Turkey, it is the long runs with high speeds and high speed corners as well that separate the men from the boys.
Hang on... you're agreeing with me?!?!...