Of course, you'd have to figure out how to define "regular HN users" and what would be a significant portion of them.
If this poll were to get that much response, it would also tell you something interesting: that HN users respond to polls in statistically significant numbers :)
So, voluntary response polls are not worthless if enough people voluntarily respond to them, but this is a tricky problem.
No. If 50% of HN visitors voted but 99% of those over 40 didn't (due to whatever confounding factor you like - say embarrasment), you'd still wouldn't be able to extrapolate from the sample to the category of "HN visitors".
eg: a gender survey in the general population that transgender respondees do not answer -- ends up with a 99% sample size (all but the transgenders) -- and thus gives a pretty significant bit of data on male:female ratios, etc..
Agreed in principle though.
"The median poll selection of a HN visitor who clicked on a poll which was on the front page on the following weekdays (and national holidays in the following countries:...) was..." etc.
If the information content is sufficiently hard to extract usefully that it would be easier to redo the sample in a sensible way, then you could call the sample worthless. (A bit like uneconomic oil reserves).