Since that's your only feature, I would assume that's what he's talking about. Opera does not support html5 drag & drop.

Atlassian uses Google Gears as an alternative for browsers that don't support it, and I think there are other tricks to support it, but I'm not sure.

Perhaps browser feature detection would be in order, if you're not already doing this?

We're going to add a classic file upload page for those who do not have a HTML5 supported browser. Thanks for the feedback.

I've never heard of modern Opera as being considered not to support HTML5. The concept is pretty nebulous though.

I see a castle and what looks like a sunset behind it. I'm not sure what I should see, but I don't think that's it.