Vitamin D was originally identified as necessary for proper mineral absorption and metabolism. Deficiency causes rickets, which results in the demineralization and weakening of bones and teeth. A modest intake of vitamin D is enough to prevent rickets. However, there is a mountain of data accumulating that shows that even a mild form of deficiency is problematic. Low vitamin D levels associate with nearly every common non-communicable disorder, including obesity, diabetes, cardiovascular disease, autoimmune disease, osteoporosis and cancer. Clinical trials using vitamin D supplements have shown beneficial and sometimes striking effects on cancer, hypertension, type 1 diabetes, bone fracture and athletic performance. Vitamin D is a fundamental building block of health.
Here's how to become vitamin D deficient: stay inside all day, wear sunscreen anytime you go out, and eat a low-fat diet. Make sure to avoid animal fats in particular. Rickets, once thought of as an antique disease, is making a comeback in developed countries despite fortification of milk (note- it doesn't need to be fortified with fat-soluble vitamins if you don't skim the fat off in the first place!). The resurgence of rickets is not surprising considering our current lifestyle and diet trends. In a recent study, 40% of infants and toddlers in Boston were vitamin D deficient using 30 ng/mL as the cutoff point. 7.5% of the total had rickets and 32.5% showed demineralization of bone tissue! Part of the problem is that mothers' milk is a poor source of vitamin D when the mother herself is deficient. Bring the mothers' vitamin D level up, and breast milk becomes an excellent source.
Stephan provided links to sources for a lot of the studies showing the relationships he notes. You can get those references on his blog.
Mark Sisson also has a post up about the sunshine vitamin.