So I war reading the other thread
http://forums.chaosgroup.com/showthread.php?74637-Real-looking-glass-(yes-again)
and made some tests I would like to dicuss with you guys.
My assumption from these tests regarding fresnel reflections. Our materials (walls and anything that is not glass) are “wrong” 
Actually this could fix some problems I red over the past here in the forum. Those are:
The sky is to dark. To less reflection. Overbright scene while the same exposure of a realworld camera gives a darker result.
First let´s look at a photo I just took from a window.
So this is how a reflection looks like in realworld.
Now I made a test setup in Vray using a Physical Sun and Sky and I ´m using just the “multiplier” of Reinhard Tonemapping to expose the scene.
The Reinhard Multiplier is the same as when using a physical camera. It´s linear. It´s easier to understand this way.
No Linear Workflow or other tonemappings where used as it doesn´t
make a difference to the problem I want to discuss.
The basic idea is to stay as physically correct as possible. Every material except the glass has a grey Vray Material with a value of 128 mid grey.
In the final rendering I want to have this grey material as close as possible to it´s initial value.. mid grey.
So this is the first rendering. As I said before A Physical Sun/Sky was used in conjuntion with Reinhard Tonemapping (Multiplier only). To get the exposure down to a level
where my mid grey material also will become mid grey in the final rendering I had to turn the Reinhard Multiplier down to 0.02.
The glass material is pretty much standard. Diffuse black. Reflections white, Refraction white, IOR 1.6.
As we can see the reflections are much to low compared to the photo. But why ? We have a physical sky. We have a physical correct glas
with a correct IOR. (some may argue at that point that real glass can´t have a pure white reflection.. but this doesn´t matter as lower reflection
values even makes the problem worse)
Now there are actually 2 non physical ways to come across this. Either you turn up the fresnel IOR or modify the falloff incase a falloff was used instead of fresnel.
Resulting in more reflections but on the other hand you´ll have a wrong falloff curve. The other way is to use an environment on this material with a much higher value
than the actual background wich also result in a stronger reflection but brings us other problems. For example when you have a window, where you can see the sky behind.
Because you cranked up the environment map it will also effect the refraction resulting in an overbright area. Environment maps at the moment can only be split global
in reflection/refraction.. not locally on a material.
Anyway for my part both ways are acceptable as long as the image looks good. But in this example we try to stay as physically correct as possible.
Now my assumption is, if the Sun/Sky is correct and if the glas material is correct as well there is only one factor left that must be wrong. All the other materials !
Let´s give it a try. So I will now turn down the intensity of my mid grey material from 128 to 64 wich means half the intensity. In exchange for that I ´ll double the multiplier
of my Reinhard Tonemapping from 0.02 to 0.04
This is the result.
As you can see, the sky is now brighter and doesn´t look as dull as before and most important we have much more reflection going on while my
grey material is still pretty much the same.
Let´s go a step further and again I take half the intensity of my material from 64 down to 32 while I double the multiplier of Reinhard to 0.08
The result.
And now our intensity of the reflection is much closer to the one of the real photo while the glas material was not touched
and our grey material still has the same value (in the final rendering) in all 3 pics.
The sky became a bit overbright but this is quite common when working linear (linear not linear failflow 
)
It would be cool to have a global switch to turn down the intensity of every diffuse component of a scene during rendering.
Because it ´s not quite fun to work with such low values in the material editor. Well in case soemone does not want to fake it.
cheers
tl:dr if you turn down the intensity of your materials in the same way as you turn up the exposure.. reflections look better.
