> I'm not trying to start a new discussion on each one of those defaults,
> but are there clear guidelines on what makes a default value better
> than another?

I'm afraid that's just wishful thinking.  Personally, I try thinking hard about
a reasonable default and avoid changing anything after that so that no user has
to worry about unreasonable breakage after updating.

> Or is it just a dozen individual discussions, sometimes resulting in
> a new default, and in all other cases the default is whichever option
> appeared first?

This sounds more like it.  Given that backwards-compatibility is such a big
deal, whatever setting appeared first is bound to set a precedent.  It requires
big activation energy to actively change that default.  Bonus: There is bound
to be less users using a non-default setting compared to those using the
default one, so even comparing user experience reports to judge the benefits of
changing the default is going to be challenging (not even accounting for
phenomena such as the silent majority of happy users or vocal minorities).