Example: Most English accents today don't pronounce an R if a vowel doesn't follow it ("non rhotic"). This is also true in accents of New York, New England, and parts of the US southeast.
However, a few hundred years ago English accents were all rhotic, like the majority of the US. I had heard somewhere that the non-rhotacism was actually adopted in the northeast and affluent south after it became popular in England and also after colonization by people who would have pronounced all the Rs. That is to say some well to do US accents kept up with changes occurring in places like London.