I think Simon Gilman has put it best. Having spent my first 20 years in America and 14 years since living in the UK, I find that the average Brit is satisfied with teeth that might be crooked, stained, even slightly rotten, if they aren't causing much discomfort to their owner. Average Americans have much higher standards of expectation in the state of their dental health -- regardless of whether they have to pay for dental treatment or not, they feel very strongly that good teeth are a wise investment in looks and health. Many visits to the dentist by average Brits happen only after something has gone horribly wrong with their teeth, whilst Americans take an actively preventative approach with semi-annual dental checkups and intervention before any problems become severe. As a result of the greater amount of attention that Americans pay to their teeth, they are far more likely to notice and comment on the state of other people's teeth.