This has always been my favorite of HN's editorial conventions. It's interesting how the tone changes with the tiny alteration from e.g. 20 Common Mistakes Java Developers Make to just Common Mistakes Java Developers Make.
It just seems less hype-oriented and attention-mongering, similar to how (if I remember correctly) HN automatically removes exclamation points from link titles.
[EDIT: Or maybe I just like that it's quirky and contributes to the particular "voice" of HN, like how the New Yorker's quirky styles for diaereses and acronyms contribute to its particular voice.]
Of course, you could argue that leaving the numbers in informs HN readers about which articles are mere gimmickry rather than carefully written essays.
In fact, in the given example, the non-numbered version of the title sounds like it should be more authoritative. It goes from an interpretation of, "a list of 20 common mistakes," to one of, "a list of /the/ common mistakes." I.e., to me at least, the second option has an implication of being a list of all the common mistakes, while the former implies that this while this is a list of 20 mistakes, there could be more.
It is the adjective ("amazing," "killer," "must-have," etc.) present in the title and NOT the number that conveys authority, completeness, etc.
Perhaps I am the exception, though, and not the rule.