But surely the condition that it's hard to make more of something is necessary but not sufficient to make that something a good "money" candidate. People expect e.g. gold to continue to be money-like mostly because of its multi-thousand year history as money. So in that very large respect, gold is quite different from bitcoin.

I am interested in bitcoin, though, because of the obvious fact that it has something that neither gold nor dollars, euros, etc. have - it can be used anonymously over the internet. So it's possible to see it becoming popular enough to be "money", which is something important you can't say about something that is merely scarce. (I have to admit that I don't think it's likely, though, and I think investing all of your savings in it is a terrible idea.)