My first entry into the DPChallenge contests was an absolute disaster, placing third from last. I ended up learning a lot more about DPChallenge and about the way my eyes work than I did about photography from it, and I learned most of that by reviewing the comments in all of the other images, rather than my own. A few people tried to be helpful in the comments on my own image, but unfortunately only told me one thing that wasn't already obvious to me. Comparing the top placing shots (and the comments on them) to lower placing shots was very educational, however.

What I submitted:

Notes on the shot itself can be found by clicking on the picture.

Basic principles learned:

1. If a single image is the only one you looked at that evening, and you have a firm memory of what the object actually looked like, my brain will perceive things as looking a lot better than they actually are.
2. Looking at just the thumbnail can also make the image seem better than it is.
3. As a conclusion from the above, never, ever, submit a photo without comparing it to other good shots at the same resolution. It was painfully obvious once I had a look at it in context that it was a bad idea, but by then it was too late.
4. As a corollary to that, if you have an alternate shot in mind, finish it as if it were going to be your final entry, even if you think up front that it won't be as good. If I had bothered to actually crop and finish my alternate idea, I would have seen at once that the technical quality of the first was too far below to enter, and would have gone with the second. Oddly, that one might actually have done well; see below.
5. There appears to be no way to remove an embarassing shot from your portfolio. It may drop off the bottom after a few months of contests if you participate every week.
6. It's actually somewhat difficult to follow the Basic Editing rules if you've gotten used to a variety of image editing tools. You find yourself reaching for something illegal out of habit. Review your steps against an open copy of the rules before you make your submission.

Things learned about DPChallenge

1. A sharp photo of a commonplace or even boring subject trumps a blurry photo of an interesting subject, hard.
2. If there are obvious flaws in an image, nobody will bother commenting on anything else. You know, little things like composition, choice of subject, what the shot evokes...
3. As a result of the above, if you have a shot that is not as good technically as some of your others, and you know already why, don't bother submitting it, since it's unlikely to draw useful commentary.
4. A beautiful picture of something only marginally matching the challenge description trumps a merely good picture of a much closer match. As long as you don't completely violate the challenge constraints, the relation of the photo to the topic seems to be mostly irrelevant.
5. Dramatic but unrealistic-looking shots trump realistic but ordinary shots nearly every time. Even surreal seems to be okay. This doesn't seem to apply to bad, unrealistic shots, which then get negative comments for being unrealistic that never seem to appear on better shots, even when they're surreal.
6. Corollary to the above: commenters apparently sometimes can't tell what's realistic and what isn't.
7. Some voters appear to randomly give ones. One of the ribbon shots got a one. Other voters appear to randomly give tens. My awful shot got a ten from someone.
8. Size matters. Anything that doesn't come very close to the maximum size in at least one dimension gets hammered, even if it's well within the upper bounds of the size limit. My entry was 530 out of 640 pixels across, and still got noted for this, which was the one thing I did learn from my comments.
9. The voters seem to like landscapes and water.

With the above in mind, I really should have entered my alternate image:

This is actually similar in choice of subject matter to the winning entries, though not as dramatic. I discarded this idea originally for not being sufficiently unique or interesting. Bad mistake on my part.

Lessons learned about digital photography and editing

1. Be very careful of anything that might be a distraction in the background. Even if your subject draws your own eye, other eyes, not seeing the concept in your head, will be bothered by anything potentially distracting.
2. A resize of a larger crop to a smaller size seems to end up clearer than just a smaller crop. I'm not sure why this is, as it seems very counterintuitive. It's possible this isn't really true, and it's just my particular test images.

I've actually learned some additional things already from my Minimalism entry, but since that contest isn't over yet, I'll save them for another day.

(Update: I cloned this off to the DPChallenge Forums, where additional comments might be found.)