OVERSHARPENED
Your photo(s) were sharpened too much.

In most cases this problem is due to oversharpening of the photo during post-processing in image editing software. Try sharpening the picture less aggressively to get rid of the jagged edges. These jagged edges are usually very visible on titles, cheatlines, registrations, or other straight lines. Selective use of masks to avoid excessive sharpening or areas where jagged lines may occur is a difficult but very effective method to avoid jagged edges. A good Airliners.Net image should be sharpened to just before the point where the titles and edges of the aircraft start to turn jagged.

This rejection can also occur when an image is well sharpened overall but one or more parts of the image showed jagged lines. This problem is generally most evident on solid lines such as wing leading edges, particularly if several lines are close together, such as gaps between the wing and flaps, cheat lines spaced closely together, or the aircraft titles and registration.

A simple yet effective method to avoid "jaggies" is the use of layers in Photoshop.

First sharpen the image slightly, just enough so that no jaggies appear, and the image is still slightly soft.
Then create a new layer, after which you apply more sharpening.
You will probably see "jaggies" appearing now. Use the "eraser tool" on those parts of the image where you see jaggies.
The jaggies should now be gone and you can "flatten" the image.

Jagged appearance on wing and flaps leading edges, the tail logo and the blue scheme curve on the plane's front part; with aircraft already pictured as often in the database as G-MONJ above even such slightly oversharpened shots get rejected. - Photo copyright © Thierry Deutsch

Jaggies showing along several lines, especially the tails and red logos. - Photo copyright © Thierry Deutsch

Clearly demonstrating the issue: 1st picture: jaggies showing the oversharpness around almost any straight line / 2nd picture: corrected softer version - Photo copyright © Paul Markman

It's another rejection reason which can be quite subtle. It's of utmost importance that you have a well calibrated monitor to spot oversharpening issues (TFTs are prone for either showing not enough or too much of it). Grains appearing after sharpening is another hint that you might have oversharpened your picture. Experience also shows that many photographers just lack the knowledge of using layer masks as desribed above; for those contributors take a look at this Masterclass thread: Masterclass: Sharpening