Oh, dear, I really didn't expect to go on this long, especially about such a minor thing! But to respond --
Hmmmmm, you asked, "GellChom, if the dress code had been business casual, what would your opinion have been on jeans?" I would not wear jeans to a business casual event. Even nice jeans are not business wear; they are for social occasions (and yard work!).
Peaches, I think you put your finger right on it. It's the negativity that's really the worst part. It's a less gracious way to try to get what they want.
The over-specificity still bothers me, though. It's just a little too controlling, a little too absorbed with making sure everyone makes Our Special Day just exactly the way we want it. The same as if the invitation said, "Don't be late" -- of course the guests are rude if they come late, but you just have to trust them to know that, not treat them like naughty children who will get it wrong if you don't spell it out. If someone wears a tie, what is the problem? As Iris points out, there are plenty of resources that people check if they don't understand what "casual" means. (And it does not mean "no ties" -- that may be what the hosts prefer, but a tie isn't incorrect at a casual event.)
"Dandy Andy's Daddy's Love," I think your post was a bit harsh. In fact, we have hosted several large events ourselves, some formal, and I understand perfectly both what goes into all that planning and expense and also that the hosts have a vision, and there's nothing wrong with that. That's why when there is any doubt at all, and if I happen to see the hostess (could be the host, but usually the hostess cares more), I try to ask to see if I can tell what she's hoping -- usually, it's the dressier end of whatever range there is on the invitation -- and I dress accordingly. I know how much that means to hosts. I also think that it shows that you consider their event a very big deal if you dress up for it at the dressier end of the range. Nor was I "pouting" because I can't wear "designer jeans" (which I don't own anyway) to a party that as it happens we can't even attend and to which I would not have worn jeans of any kind, even as part of a rather dressy outfit.