As you mention mixed content and DNSSEC in one sentence, I must ask how do they relate?
I've never understood DNSSEC (although I've never took the time to understand it properly).
The mixed content is a concern yeah. In ie8 (higher ie's as well? don't know) you get a whole popup warning, which annoying. And it's not _really_ secure anymore then (although afaik not less secure than no ssl).
The certificate is 12 pounds or so. Hardly a burdon!