I like TL;DR. It reminds the author of the basic writing principle that nobody wants to read anything you write, they're doing you a favor by reading the first two sentences and if you're still saying boring things by the third sentence that's it. Writing is terrible in proportion to how its circumstances cause writers to ignore this principle, for example school textbooks that children are forced to read, or journal papers that adults are either forced to read or that aren't being written for the sake of producing understanding in anyone.

You present a strong argument that "TL;DR" is an excellent thing to keep in your mind while writing. It is not a terribly good reason to write "TL;DR" as the first section header in a post.

"Too Long; Didn't Read" is just wrong. It doesn't mean "short summary of the key point which can be used to establish whether to bother reading the body". That's what we have words like 'summary', 'abstract', 'introduction' and 'synopsis' for.