One reason we see so many tl;dr comments even on a very technically inclined place like HN is that articles very often bury their content. We want a thesis out front; we don't want to be strung along for 1600 words and then find the interesting bit between 1601 and 1700.
However, I also think that the phrase “too long; didn’t read” represents a dysfunctional attitude that turns this blessing that is the abundance of information into a curse: if you spend three hours reading 100-word abstracts (let’s say, 300 of them), at the end you will probably have learned almost nothing. If you instead spend three hours reading 1000-word articles (let’s say, 40 of them), you will probably have learned a small amount, which is an improvement. If, by contrast, you spend three hours reading 50 000 words (say, a few chapters of a Murakami book), or a quarter of Olin Shivers’s dissertation, you will probably acquire some new ideas that will stay with you for the rest of your life. There’s a point at which this becomes counterproductive; if you read a book in a day, it won’t stay with you as well as if you read it over the course of a week, and the week also gives you time to reflect on other connections.
My thesis: being able to focus your attention on interesting things is only valuable if you then do actually focus your attention on them. The attitude expressed in “tl;dr” is that focusing your attention is itself undesirable.
It's not about low attention spans, it's about there only being so many hours in a day. It's about helping people find what they want to read because there is just too much content being produced every day.