These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Colbert Bump: Inverted when Jay Leno permanently took the lead over Letterman in the ratings after his 1995 interview with prostitute-solicitor Hugh Grant. This came in the wake of Letterman's disastrous stint as host of the Academy Awards that year, where his style of humor clashed horribly with the tone of that event.
Seasonal Rot: Dave has been on broadcast TV for almost 30 years. In the mid-2000's, the quality of the Late Show has declined slowly. However, every so often, Dave will have his standout moments, such as the infamous Joaquin Phoenix interview or the mileage he got from the Jay Leno-Conan O'Brien controversy.
Seinfeld Is Unfunny: Letterman is undoubtedly the most influential figure in American comedy over the last three decades, and as this article points out, he pioneered a lot of the ideas and attitudes that now saturate the Web. But because his influence is everywhere (not to mention Seasonal Rot) he seems really passe these days. His early NBC shows come off a bit dry and awkward, and his peak period (the last few years at NBC and the first few at CBS) isn't easily available (though there's a fair amount of varying-quality YouTube video).