TL;DR is, at least at first glance, an abbreviation for Too Long, Didn't Read.

What it really means is that you have encountered a person with a pathological attention span. They are simultaneously saying that they have failed to bring adequate reading skills to bear, combined with the intent to cut someone down for "elitist" behavior. What elitist behavior? That of writing a literate, informative post. Yes, really.

So when you encounter TL;DR, you can just ignore it. I put no small amount of blame upon the "sound bite" environment created by television. The erosion of the ability of much of the populace to deal with even a mildly extended presentation has closely followed television's failure to present any such thing.

No discussion of this would be complete without a nod towards courtesy. If someone addresses an issue at length, this is an offering of information to everyone else. If you legitimately don't have the time to read it, then the answer is not to say "You should have written a sound bite." The answer is silence until you have read the material.