This would be a great way of making things pretty in userpages and other important/high-traffic posts. Just let the parameter for the tag equal what would be written in the style attribute of the HTML tag.
[span=background:url(http://puu.sh/1aIt0) bottom repeat-x;font-weight:bold;color:#800000;]WHAT COULD POSSIBLY GO WRONG?[/span]
becomes
<span style="background:url(http://puu.sh/1aIt0) bottom repeat-x;font-weight:bold;color:#800000;">WHAT COULD POSSIBLY GO WRONG?</span>
Divs could be used for cool things like boxes and columns.
Force position:relative to prevent someone from drawing a giant black box over the forum page though.
[span=background:url(http://puu.sh/1aIt0) bottom repeat-x;font-weight:bold;color:#800000;]WHAT COULD POSSIBLY GO WRONG?[/span]
becomes
<span style="background:url(http://puu.sh/1aIt0) bottom repeat-x;font-weight:bold;color:#800000;">WHAT COULD POSSIBLY GO WRONG?</span>
Divs could be used for cool things like boxes and columns.
Force position:relative to prevent someone from drawing a giant black box over the forum page though.