>>I understand NULLS have a special purpose... but I think I'm starting to abuse it. For example, if we have a text comments area that saves to a varchar field, would it be proper to use NULL if the user left the text area blank, or would it be proper to use a blank string?
>
>I use NULLs for numeric columns. I have business rules that require me to know if a user entered 0.

I think nulls for numeric columns is a fairly easy choice as I think your business rule is common.