Both Seinfeld and Letterman, I believe, require a high sense of irony to be appreciated. The two shows don't necessarily play to the exact same sensibility, but there is quite a bit of common ground. Familiarity with contemporary US foibles, vs a lack thereof, might also be an important factor.

For those who *like* the Seinfeld series, you'll probably really enjoy Larry David's HBO series "Curb Your Enthusiasm," the early seasons of which are now available on DVD. Larry was co-creator of "Seinfled" (with Jerry Seinfled, of course), and the real-life model for the character George Costanza.

I thoroughly enjoy all three of the above, but have no need to (nor hope of) converting the unimpressed. Letterman, in particular, is such a long-familiar character by now -- like an old family friend who visits our living room most weekday evenings -- that I'm usually ready to laugh at just about any stupid thing he starts to talk about.