I love IOTD. I don't go out looking for it, but it frequently catches my attention on the right pane and I think that's a good thing. However, one major gripe I have with the feature is that these often wonderful looking screenshots are displayed at such appallingly low resolution in the preview window that it takes away most of the magic. Can't the default display size be made larger? When I click on it, I'm okay with it going full screen for me.
Anyways - that's not my suggestion. My suggestion has to do with interactivity.
Most of the IOTD candidates don't get many comments (if any). To be honest I haven't kept track, because the comments section seems to be pretty dud. So my suggestion is to alter it a bit to take away some of the chore of actually having to write up a sentence (or rather add to this option). If you're a user of ViewBug (requires registration/linking with FB to get access to the main features, I think), you'll be familiar with the awards system whereby each member can grant any image one of a variety of awards (eg "Jaw Dropping", "Top Choice", "Outstanding Creativity" etc). I don't think there's a limit to the number of awards a member can give out to different images, but it does present a very simple and constructive way of focusing community feedback. Don't want to leave another lame comment that says "great!"? Just give the image an award.
This might be one of those in-your-face features here on GD that largely replaces the comments section in IOTD. It would also allow images to be further ranked - not just by vote, but by type or style.