Design your form to reflect your users' goals
Nearly all forms exist because a user wants to do something, and needs to supply data to achieve that. Tom Mollerus has a great summary checklist on usable forms that focuses on designing your form so that you help your users to meet their goals.
Do you still need those answers?
Many times, a question is put on a form to fulfil a current need. Then time passes, the need passes, but the question stays there.
Example: a police form that asked police officers to record race. No longer needed, potentially offensive.
Example: an 11-question web contact form that asked for unnecessary details such as street address and fax number. Reducing the form to the four most important questions resulted in sharply increased conversion.
Losing a customer over a detail
Some forms are a mere detail in an overall process. For example, think about the 'unsubscribe' form on a typical marketing list. Chances are that this hasn't been the main focus of your development effort - and nor should it be, because if that bit works well and the rest is rubbish, then so what?
Only - it is a detail that is worth getting right. And in some jurisdictions, it may be illegal to send marketing emails without an appropriate unsubscribe process, as Colin Watson (Clerkendweller) points out in his blog post: Marketing email opt out? Let's make it very, very difficult
Each extra question adds burden to the form
It may seem like just another innocuous question, but each extra question lengthens your form and makes it less appealing to fill out. Bored, annoyed users are less willing to finish the form, and less likely to take time to think of accurate answers.
For example, as David Hammill points out, the question "How did you hear about us?" is often seen on registration and other forms. But it annoys users because often they can't remember how they heard about your organisation, especially if you're a well-known brand.
