The best posts are ones that say something surprising (e.g. not just a reporter writing a routine story about a familiar topic), and say it in a convincing way, with depth of argument, and numbers, if applicable.
Posts about how to do things oneself, and how things work, tend to be particularly appreciated. This is an audience that likes to know the details.
Posts don't have to be about hacking, so long as they talk in detail about something novel or surprising. Though arguably anything that talks about the internal details of how something works is about hacking, in the broader sense.
http://news.ycombinator.com/item?id=411994
http://news.ycombinator.com/item?id=406885
http://news.ycombinator.com/item?id=418776
http://news.ycombinator.com/item?id=414330
http://news.ycombinator.com/item?id=418329
http://news.ycombinator.com/item?id=414226