Letterman is essentially the same as any other talk show. The distinctive feature was Letterman himself.
The Vizard show slavishly copied his mannerisms (eg. walking around with a cup at that start of the show) and the timing of the segments. It was all very strange until I saw an episode of Letterman. Of course someone who ended up being forbidden for running a company for ten years after being caught at insider trading is exactly to sort of person that doesn't care about copying someone else's show that has worked elsewhere. Few people in Australia had seen Letterman back then.
Top Gear however has a wider international reach than most things, it's broadcast just about everywhere on the planet.