There is no such feature, but you can build it in with a little VBA code:
Private Sub Application_ItemSend _
(ByVal Item As Object, Cancel As Boolean)
If Item.Subject = "" Then
Cancel = True
MsgBox "Please fill in the subject before sending.", _
vbExclamation, "Missing Subject"
End If
End Sub
For a more elaborate version that also checks for expected attachments, see
http://www.outlookcode.com/codedetail.aspx?id=553