Here's the thing, though: they left "onions" off the description of that pizza in the menu, and we had to send it back to get it without onions, because just picking them off is not really the thing when you didn't want the ensemble to taste like onions at all.
This time, I ordered a chicken dish. And this time the thing they didn't mention on the menu was capers. Luckily I both like capers and find them appropriate to this chicken dish, so no biggie--but timprov had been thinking of ordering it, and he does not like capers. And again: you can't just pick out the capers in something that's made with capers and have it taste the same. They're very flavorful. They flavor their surroundings right away.
Here is my question. Which ingredients, for you, seem not worth mentioning on a menu? I can see the argument for onions, even though I deplore it: onions are very, very commonly used. In fact, when we sent it back to ask for it to be remade without onions (yes, we tipped the waiter substantially), the waiter said, "I think I didn't hear you say you wanted it without onions." I said, "I didn't. Because the menu doesn't say it has them." And he looked very confused, and I suspect this is because he didn't consider them notable or worthy of mention on the menu--it was just reasonable that there would be onions in this dish.
So what are your limits there? Which ingredients do you consider major in what things--what goes on a pizza by default, for example?
I assume that those of you with severe food allergies would ask to make absolutely sure, but we don't have severe food allergies, we just didn't want onions on this particular item.