My take on the letter is that continued dabbling with "gender blurring" fashion trends can eventually result in a form of private reproof.
They are saying "if your appearance doesn't match what's between your legs we don't want you going door to door". Gender blurring, to use the letter's terminology, has emerged in fashion on and off for many years, but at least from my perspective it is bigger now than it ever has been. No big surprise it's being singled out. It's also a hot button political issue, they don't want representatives to appear to be on the "wrong" side of it.
I think what gets to me regarding fashion is how mixed the WTS is on fashion in general. You get the sense that they truly want conservative dress, but are hesitant to demand it, probably for fear their membership will appear like a regimented cult.
Another issue is that JW faith provides few avenues for personal expression, and too much push against personal fashion may actually constitute something the r&f won't tolerate, at least in western countries. So the Watchtower rides this middle ground, always strongly requesting conservative dress but not demanding it.
To put it another way, what is holding the WTS back from instituting a dress code? Why don't they just pull the trigger?