For example, I tested tldr.it on a recent HN submission, Programmers: How to Make the Systems Guys Love You (http://news.ycombinator.com/item?id=1800839), and it seems to select the first paragraph or so of the article (depending on whether you select the short, medium, or long tldr).
However, so many authors these days start out with a long, rambling preamble, instead of the venerable Who/What/When/Where/(Why|How), that the first few paragraphs don't necessarily provide a real tldr-type summary.
Which is the case with that particular article - the first few paragraphs don't help me decide whether to read it or not, and the entire article is quite long, with some good nuggets further down in it.