Excuse my language, but that's bullshit, and it makes me angry.

Firstly, vitamin D deficiency is not the most common medical condition in the world. I don't know what the most common medical problem is, but of serious diseases Hepatitis B affects around 2 Billion people (http://www.infoplease.com/ipa/A0903696.html).

Secondly, even if you believe vitamin D deficiency is a serious problem then you may be surprised to learn about places such as China, India, Indonesia, Brazil, Pakistan, Bangladesh, Nigeria etc etc, all of which have very very large populations well south of the 33rd parallel. In fact, by far the majority of the worlds population lives south of the 33rd parallel north.

Thirdly, where I live (Australia) one of the major causes of death is cancer, specifically skin cancer. I know at least one person who sends her kids out to play in the sun in summer after hearing this (without sunscreen).

In my view Vitamin D deficiency is a minor problem, easily fixed which affects a small number of people. Blowing it up into a major issue has the potential to cause a much bigger problem (skin cancer) in a similar way to how vaccination scares caused public health risk.

As I understand it, one of the main reasons indigenous Europeans evolved lighter skin was to allow vitamin D synthesis in a cloudier climate. An indigenous Australian in Europe is more at risk of vitamin deficiency, while an indigenous European in Australia is more at risk of skin cancer.

To be more explicit about it being "easily fixed", here's a quote:

Asked for a general recommendation, Dr. Holick suggests going outside in summer unprotected by sunscreen (except for the face, which should always be protected) wearing minimal clothing from 10 a.m. to 3 p.m. two or three times a week for 5 to 10 minutes.

That's all it takes to fix the "problem"

Yeah, certainly throws that Aussie mainstay "between 11 and 3 sit under a tree" out, and that's a dangerous move if you do live in Australia. Since moving to the northern hemisphere I really feel as though the sun here is less intense, but I'm also a lot further north (Belgium) than I was south (Sydney) of the equator, or so the longer summer days seem to imply. It certainly doesn't feel as though it will bake your skin off like it did back home, though old habits die hard and I still go out slathered in cream :)

No, you aren't feeling things, the sun in the Northern Hemisphere (at least Europe) is less intense, even at lower latitudes like Italy.

I only got sunburnt once in Europe, and that was after being the sun all day with no sunscreen or hat. It took literally hours to get sunburnt. Here, I think 30 minutes is about all the average Australian can take, and for a recent arrival from Europe, 15 minutes will do them in, and 2 or 3 hours will put them in hospital.

My personal theory is that there is a lot more airborne pollution in Europe, and because of all the air traffic a lot of high-level cloud gets laid down by planes. You'll note the sky is never as deep blue as it is in Australia. I also think that because of the tilt in the Earth's axis, the SH summers receive a more directly overhead sun than even on the summer solstice in most of Europe. Finally my theory rounds out that, in Europe, periods of hot sunny weather seem to coincide with long periods of stable weather with little wind, when the pollution quicklyk builds. In Australia, in summer (in much of the country) the really hot days come during the thunderstorm period, when the high wind and heavy rain blows all the pollution away, and the next day is scorchingly hot.

Of course, all just random theories but based on a lot of observation.

I suspect you're right, especially about the pollution. When landing in a plane into Brussels (in particular) the brown blanket is quite obvious. Not so much when you're on the ground, though you're right in that the sky never seems quite as blue (though I'm willing to accept a certain amount of home bias there).

I mostly agree with you but I think Australia is a special case. We've got a largely white population living in some extremely hot areas so the risk of skin cancer is far greater than the risk of Vitamin D deficiency. For people in northern latitudes those risks are going to be reversed.

Many people with pets kept indoors would be very surprised to learn that these animals can needlessly suffer from painful arthritis in old age due to vitamin D deficiency. Dogs and cats don't get much sun on their skin, but they secrete oils on their hair which produce the vitamin when in sunlight. The animals then ingest it when they lick themselves.

Some people that thing they're getting sun while driving don't realize that auto windows are designed to block UV.

For people living in the 3rd world it is appalling how crazy and unrealistic is the world view of people that never saw someone with schistosomiasis, malaria, sleeping disease, tuberculosis, ...

Even for developed countries, what about the side effects of overweight and obesity? According to the World Health Organization the 2 most common causes of death are heart attacks (ischemic, in particular with 12% of deaths) and brain circulatory diseases (e.g: stroke). Overweight/obesity is an important factor for both.