In my defense:
1) There's definitely an element of "if the user can’t find it, the feature doesn't exist" with downvoting here.
2) In practical terms, downvotes are so extraordinarily rare on HN that they almost don’t exist.
Anyway, I'm encouraged that downvotes exist on HN, but the threshold could be considered so absurdly high that they're nerfed into oblivion.
And they don't exist for article submissions, which is problematic for all the same reasons I listed in the blog entry.
As for your assertion that not disclosing the karma threshold for downvoting is 'bizarre', I'd argue it's useful not to provide an exact number. Enough video games have 'achievements' listed out for people to obtain... I'd rather have people participate honestly than just to gain more 'abilities'.