Basically, users will skim over forms they have filled in, before the dreaded hitting of the submit button. We do it with paper forms too, but we can actually measure real living human beings doing it on web forms because of the eyetracking usability research, which for some reason either isn't popular with printed materials, or maybe is just too hard to measure there?
A search or two-input login is probably okay. E-commerce form with 10 or more questions? No way. Always show some kind of label so users can double, triple and quaduple check that what they have filled in matches what is asked.
This is double important when you are sending back error messages in your form validation: do not remove what they have typed in, but highlight it or point out that it is incorrect, tell them why and how it is incorrect, how they can fix it, and here a label is of utmost importance. For some questions I would imagine if someone got it wrong once, add some sort of additional hint on validation, like an example (the kind of thing placeholders were meant for, but of course we can go further: pictures can be quite awesome. I've seen them now for credit cards (both the numbers in the front AND the so-called security number in the back) and licence plates (which my own company used to great effect).
Once, when I worked in the medical industry, we had a (paper) form that asked for a number on a pass. The name of the pass was pretty standard from hospital to hospital, but still too many people had no idea which pass this was (some folks had separate passes from their insurance companies, from their doctors, and their specialists). A pic of our pass helped a lot.