>>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?
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>I use NULLs for numeric columns. I have business rules that require me to know if a user entered 0.

There is no question there is a place for nulls, and that's a good example. But I don't believe they should be overused. For example, a typical address provides for two lines of street address, the second typically being used for suite number, apartment number, etc. If it is not specified I consider that an indication that there isn't one, not that its value is unknown. In fact I would be hard pressed to think of a character field that is not required by database rules (street address line 1, for example) that should be nullable.