This is actually quite an interesting topic. If you go and research "glare" you'll find quite a lot on the subject, and how different people are affected differently by the different types of glare.
If you go to research "reflection of dashboards in windshields" you'll find quite a few discussions on many forums on the subject as well--it is an equal opportunity issue affecting many cars and many people in different ways.
This much is pretty well known--
dark dashboard material (i.e. a black dash or one with a dark dash mat) creates less of an issue than lighter, more reflective colors.
matte dashboard material (as opposed to Armor-All shiny) creates less of an issue than shiny, reflective surfaces.
Angle of dash and windscreen make a difference, as does angle of sun--even allowing for instances as Cees points out of having the issue on a flat-light overcast day.
Inside and outside cleanliness of windscreen makes a difference, too--film, dust, dirt, etc. on the windshield provides a projective surface.
Polarization helps big time.
To that last point, this morning I moved my daily driver out of the garage and checked for this reflection. Sure enough, it was there, but it was predominantly in the lower part of the windshield, thus not too bothersome. I looked through my polarized sunglasses, and voila, the reflection of the dash disappeared 100%. To see the effect of the polarization, I removed the glasses, looked through them and rotated. As the glasses rotated, so did the effect of the polarization and the reflection began to come into view once again. At 90 degrees of rotation, the reflection was back 100%, albeit damped by the darkness of the lens. When the polarized sunglasses are at 0 degrees of rotation and manufactured properly, the reflection/glare is gone.
Plenty of answers, some easy, some not so easy.
Some don't want sunglasses--so polarization won't work.
Perhaps to some the only thing worse than "ugly" sunglasse styles would be a dash mat. When I lived in California plenty of people had them to cover up cracked dashboards they did not want to replace...like the one I had on my old VW Golf.
The corrolary to that is having a dash mat will prevent the sun from cracking it in the first place!
Newer cars tend to have newer plastics and require almost religious and regular cleaning of the inside of the windscreen. I do mine all the time, and am never satisfied. I still get all kinds of streaks, and not convinced I've got it all clean. I've tried everything.
As Alfred pointed out about hazed/pitted windshields, that's yet another issue and not everyone wants to invest in a new one!
Aside from all the technical solutions, there is the biggest and that is the human factor. Some people's eyes are just way more sensitive to this interference in the visual path than others, and some are able to train their eye/brain to ignore it. Personally I can ignore a lot of it, but what I cannot ignore is direct glare created in my eye. It does not bother a lot of people but affects me daily in things like where I sit in a restaurant or at a table, pushing away direct light, etc. I had to close the blinds in a restaurant on Friday night, and at home there are certain places I cannot sit at my tables just because of direct glare. My family is not nearly as bothered.
As I said, a very interesting topic. Now, I guess I have to clean my windshield AGAIN!
