I suspect the problem has to do with this code, generated by the Suffusion theme:
<style type="text/css"><!--/*--><![CDATA[/*><!--*/
...
/*]]>*/--></style>
According to https://hixie.ch/advocacy/xhtml, this is the correct way
to embed CSS in an XHTML document which may be handled as either HTML4 or XHTML. I’m not expert enough to comment on whether WP-Minify should be able to handle that or whether the problem lies with Suffusion.
But when I remove the CDATA lines, IE displays the minified page.