It was often to be seen on SA under some teenaged nerds verbose essay about how much the people at their high school sucked. In that way, it was the original "cool story bro."
This original meaning is why the following image is funny: http://c.cslacker.com/3910l.jpg The joke doesn't work with the new usage.
At some point it flipped around and became a way of expressing one's tiny attention span.
I first noticed it on reddit years ago, appended to a lengthy comment. However, it seems that it may have originated in Wikipedia article discussions (according to Wikipedia...) as a shorthand _response_ to a lengthy comment. Perhaps, there was a transition period where someone wrote a lot of text, realized that the response would be a "tl;dr", and then just added it themselves instead.
{{point}}; {{elaboration}}
tl;dr: {{point}}
For what it's worth, I take that form of "tl;dr" as emphasis, not as a literal tl;dr.
This.
I've been inured to using it in that way as well. I see the criticism above as a valid one though.
* tell me what you're going to say
* say it
* tell them what you said