Beyond checking whether required fields have been filled, or e-mail addresses are legit, some advanced field validation would be a plus.
Such validation types might be:
integer
decimal
alphanumeric
date (in a format MySQL expects it)
min/max data length (i.e., for zip codes or phone numbers)
This type of validation would really cut down on the amount of garbage data inputted by users that makes sorting and reporting challenging due to inconsistent data entry.