The Mermaid makes a very good point and I am sure this also passes on, unintentionally I am sure, to some of the lounge angels tasked with the unenviable job of enforcing this policy. Even with QF attempting to make the policy more clear the fact that there's a debate here about what is and isn't acceptable just shows how subjective it can be to different people, so how is one lounge agent (who is human and has their own ideas of this) to stay consistent with what others do given there's so many variations of footwear.
It makes me think of the age old community "standard" or (custom) if you like of the differing standards of wear for males and females - eg formal wear for men usually means a suit and tie minimum, and probably extends to black tie, but there's little variation there.. for the ladies, in general, it can be dresses (of differing lengths and designs - eg high cut, low cut, sleeveless, etc etc) and it's oft been that women appear (to me as a male anyway) to have far more range and choice in this sort of situation. I mean a male.. what your variation is tie or bow tie, the colour or design and if your jacket is single or double breasted.. but what else?
So yes, I can see that one's gender can, in some cases, vary one's perception of such things... and again drags a policy like this into a very subjective area.
Anyway I'm a male who isn't repelled by thongs or sandals or whatever. It's none of my business what someone else chooses to wear (though like others I think rubber thongs on an aircraft is hardly practical, but that's up to them). As I've long posted it's all about the behaviour.. or to paraphrase another well known saying "It's not what's on the foot that matters, but what you do with it..."
my 2 cents