Here's what's wrong with the golden rule:
Different people like to be treated differently. The way John likes being treated may be an annoyance to me. So if John follows the golden rule by treating me the way he wants to be treated, he would be a pain in the ass to me.
The golden rule ignores human individuality. It leads one to act presumptuously by projecting ones own preferences onto others. The Diamond Rule is superior: "treat others the way they would want to be treated."
Imagine being the guest in the home of someone of some exotic culture and he offers you what is considered the delicacy in his culture and which he loves. With the Golden Rule mindset, your host expects you to love the meal because he loves it and is giving you with the expectation that you will love it too. With that mindset, your host could easily be offended if you genuinely do not like the meal and stop eating after the first bite. The Golden Rule facilitates such taking of offense because it projects your own preferences onto others. By contrast, if the host had the Diamond Rule mindset, he wouldn't presume to project his own likes onto you. He may offer you a small sample of their delicacy to see if you like it, and if you don't he wouldn't take offense because the Diamond Rule acknowledges that we have different preferences and seeks to know the preferences of others instead of presumptuously projecting our own preference onto them.
The Golden Rule has a form of love that proves to be somewhat superficial. The love of the Golden Rule is not genuinely interested in the person - for it does not seek to ascertain the person's preferences so as to know how to treat them. It overlooks this because it is more interested in proving itself righteous and so presumes, egotistically to project the giver's own preference onto the person.
By contrast, the Diamond Rule shows personal interest by ascertaining the preference of the person so as to know how to treat them. It shows genuine personal interest instead of focusing on proving the lover righteous at the expense of overlooking the preferences of the one being loved.
The Golden Rule is proof that the book of Matthew is not the inspired word of a superior all wise God whose thoughts are superior to our own. In fact, the Golden Rule is not even original to the bible but has been expressed by others living before the time of Jesus.
The Golden Rule is what some might call a "deepity" - a pithy saying that has an air of charisma to it but upon closer scrutiny proves to lack an intelligible meaning or the great depth of insight or substance it pretends to have. Charismatic sounding slogans tend to be like that. True wisdom is often very lacking in charismatic appeal.