I have two feature suggestions that I have thought of for custom fields (so far!)

1) Required fields - Enable a custom field to be marked as "required" so that when the user was creating/editing a post in a category the particular required field belonged to, they would have to value the custom field before being able to "publish."

2) Enhance DateTime custom fields - Maybe create a DateTime picker sort of control, or at least some javascript validation code to ensure the data entered in a DateTime field was an actual date and time, as opposed to junk data or a malformed date and time.