'tl;dr' is often a troll response to a long post that that someone has obviously spent a lot of time on. Bonus troll-points if the long post was in response to another troll.
Example:
poster1: only retards use vi, notepad rules
poster2: huge list of reasons why vi is better than notepad
I was quite peeved when I first saw a "tl;dr" comment concerning one of my blog posts. My thought was, and still is somewhat, "if you didn't read it, how can you say it was too long for what it needed to cover? Why do you feel the need to tell others that you have the attention span of a fly?"
We already have terms like "summary", "digest", and even "précis"; why create a new term imbued with snark?
But not always. You have to consider how it was used to tell whether it was meant with ill will, the term tl;dr on its own doesn't necessarily tell you enough.
Example:
poster1: only retards use vi, notepad rules
poster2: huge list of reasons why vi is better than notepad
poster1: lol tldr