I have issues with gender determined etiquette - where there is a different standard of behaviour for men and for women based soley on their gender. Things like the man having to open the door, or who goes into the elevator first, or addressing envelopes as Mrs Bob Jones, unless the woman has specifically indicated that she wants it. This is particularly vital in a work environment, where the gender based rules can clash with the workplace etiquette rules.
I really wish you could put 'no gifts' on an invitation and have it stick. The problem comes with situations where you aren't sure you should bring a gift. For a wedding, you usually do, but it's silly when someone is reluctant to host a party to celebrate a milestone or achievement, purely because people will think their fishing for gifts.
The couple rule has irritated me in the past, unfortunately the fundamental issue isn't the etiquette rule, it's the inability to find a polite, non hurtful way of letting someone known that while you like them just fine, you'd rather swim naked with pirahnahs than spend an evening in their partner's company.
I dislike most fashion based rules - white after labour day being a classic example. There is etiquette based reason for them - wearing white isn't going to hurt someone else, or cause a misunderstanding in a social situation. It's a meaningless, outdated rule that, in my experience, is usually invoked to make someone else feel gauche. Ditto for a lot of the wedding hoopla - a white dress, pew bows, matching attendants, a full sitdown dinner, dancing, drinking, may be traditional, but are not requirements.
For things like relative hosted showers and second baby showers - I couldn't care less. As long as the shower is tastefully organised and not a blatant grab for money and gifts I'm happy. Mind you, organising your *own* shower is still tacky, as are showers for aunts, new pets, graduations, etc.
I don't have a problem with registries as long as they are only used for weddings, are discretely mentioned (website okay, invitation not), have a range of prices, and are mandatory. Registries for anything other than weddings (baby showers, birthdays, graduations, first communion, divorce) are still out.