Gstreamers's textoverlay element has a lot of properties that are currently not controllable from totem. It would be nice if totem could allow changes to them, so that the appearance of subtitles could be fine tuned.
It doesn't have to be in the gui, I see how it could clutter totem's minimalistic interface. But if there could merely be gconf keys, that would be enough. Here are the properties:

  shaded-background   : Whether to shade the background under the text area
                        flags: readable, writable
                        Boolean. Default: false Current: false
  halignment          : Horizontal alignment of the text
                        flags: readable, writable
                        Enum "GstTextOverlayHAlign" Current: 1, "center"
                           (0): left             - left
                           (1): center           - center
                           (2): right            - right
  valignment          : Vertical alignment of the text
                        flags: readable, writable
                        Enum "GstTextOverlayVAlign" Current: 0, "baseline"
                           (0): baseline         - baseline
                           (1): bottom           - bottom
                           (2): top              - top
  xpad                : Horizontal paddding when using left/right alignment
                        flags: readable, writable
                        Integer. Range: 0 - 2147483647 Default: 25 Current: 25
  ypad                : Vertical padding when using top/bottom alignment
                        flags: readable, writable
                        Integer. Range: 0 - 2147483647 Default: 25 Current: 25
  deltax              : Shift X position to the left or to the right. Unit is pixels.
                        flags: readable, writable
                        Integer. Range: -2147483648 - 2147483647 Default: 0 Current: 0
  deltay              : Shift Y position up or down. Unit is pixels.
                        flags: readable, writable
                        Integer. Range: -2147483648 - 2147483647 Default: 0 Current: 0
  wrap-mode           : Whether to wrap the text and if so how.
                        flags: readable, writable
                        Enum "GstTextOverlayWrapMode" Current: 2, "wordchar"
                           (-1): none             - none
                           (0): word             - word
                           (1): char             - char
                           (2): wordchar         - wordchar