form design

Debates about web form design

I'll shortly be giving a talk that plans to address some of the most commonly debated elements of web form design.

I know what questions I get asked over and over again, but want to make sure my view isn't biased. So I'd like to hear from you. 

What debates — about competing approaches to the design of web forms, not general considerations per se — have you been involved in?

EITHER mail OR phone required in a form?

I have a client that is requiring EITHER a phone number OR email in exchange for some valuable whitepapers. Has anyone experimented with conditional validation that requires one of the two fields?

I wonder how to even inform users that we need one or the other. I could just as easily require email and make phone optional if this is a bad idea. My only thought is that users who would rather receive a phone call might not want to provide their email address and abandon the form.