I think it is rude to be outwardly ungrateful for a gift - one should always be cheerful and thankful. But I also think it is self-centered (I am struggling with the term I mean) to think poorly of someone because they were not blown away by your "thoughtful" gift that is something they don't want or need. Like the towels in Lynn2000 example. It would be silly to be offended that someone else's taste is not the same as yours, and that the brown brand Y towels were returned and not used.
I didn't see anything in Lynn2000's post that even suggested that the giver of the towels was "offended that someone else's taste is not the same as [theirs], and that the brown brand Y towels were returned and not used."
I understand that Amy is very particular, and at least in some cases for very good reason. But did she consider the possibility that the givers of the towels and baby supplies did not even see her registry and just picked out what they thought was a nice gift? And that they couldn't care less if she exchanges things -- indeed, I would imagine someone buying towels is thinking, "I don't know what color their bathroom is, but these really nice towels come in lots of colors, so I'll get them and they can exchange the for the color they want, or anything else they'd prefer. It'll be more trouble to wrap than a gift certificate, but it's nicer to receive." (What is the big deal about having to exchange things? I never heard anyone complain about having to make a trip to the store when they received a gift certificate. Well, if you think about it, a gift you are exchanging functions pretty much like a gift certificate.)
If you can't use some products because of allergies or something, and they can't be returned, then regift or donate them. I cannot imagine that the majority of items were in that category. If it's just not your preferred brand? Sorry, that is what I call an ungrateful SS -- the same as if I choose a sweater or novel for you that isn't by your favorite designer or author. We aren't entitled to receive only gifts that 100% hit the mark. Any time we think something like, "Why didn't they just get me what I wanted? Is that really so hard?" the focus is way off the real meaning of a gift and onto our own sense of entitlement.
This kind of discussion seems to rest on the premise that if a registry exists, guests may not be
required to purchase only gifts listed (although they really
should, if they want to be considerate), but that they
are required to consult it, and therefore if they buy anything else are substituting their own preferences for the recipients'. And that really bothers me. Gifts are an expression of the givers' good wishes and generosity, not the performance of a duty to fill in someone's shopping list.