Numbered lists imply completeness, omniscience etc. - it's a literary gimmick, both in terms of attracting the lazy reader and in giving the writer a convenient structure they can fill out easily, with the last few points usually being fluff. A famous example is the '7 habits of highly effective people' followed by 'the 8th habit' (strangely, not titled 'oh wait, I forgot one').

Of course, you could argue that leaving the numbers in informs HN readers about which articles are mere gimmickry rather than carefully written essays.

I disagree with this. At least from my own perspective, numbered list imply one thing: that I can expect a specific number of items.

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.

It is exactly because the "top x" title is a gimmick that I want it faithfully included in HN titles. This way, I know to not click on the link in the first place, as I typically avoid these types of list articles.

I'm not sure exactly, but I think it's mainly because it does away with the overused blog-headline cliche of the "Top X" list. It puts focus back on the content rather than the structure, and makes the headline less obtrusive when listed with all the others.

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.]