People scan the forms, they hate forms and try to fill them out as quickly as possible and get on with more important things. Short labels at the inputs make reading the form more efficient and help the user think less about filling it in.
If you feel that the short label does not reveal the meaning of the input field, you can always add details to the hint. Also, of course, you should not shorten the names of labels making them less readable and clear.
Often, you will find that the form's placeholder simply duplicates the field label. This makes little sense. In that case, the placeholder is not needed at all and just creates noise. The placeholder should show the expected data or format.
Well-designed placeholders can make it so that users will read them exclusively, not the input fields' labels, which will speed up understanding and filling out the form.
The placeholder is the only option to point to the input field without a label on some forms. For example, in a subscription form, the label is less necessary.
Use masks in input fields where special formats are required. Do not limit the user's input options. Typically, most users insert data using copy/paste. In any input field with a mask, users can insert their data and your form should take care to put spaces or hyphens between credit card numbers.
The computer must do its job and be able to recognize phone numbers with spaces, with hyphens, with brackets, with a country code, and without a country code. And be able to determine, regardless of the format, the correctness of the data entered.
Don't make users struggle with input errors. They will just leave and won't continue filling in the form. You need the data, not the user, so it's worth taking care of recognizing phone number formats and converting them on the service side into the format you need. And so with any data entry field that requires a special format.
Let users decide which password to use for your product. It's their responsibility. And, be sure to let them see if they entered their password correctly if they want to.
If you ask for an email in sign up forms, you should not be asked to re-enter the users' password. If anything, they will recover it by email. The only thing worse is when you ask users to repeat the email they entered. That's just ridiculous. Users just copy/paste it with annoyance.
