Lack of Composition. ...
To me, it means Unappealing to the Eye.
Ok, you say, but what does that mean? How can I make my images more 'appealing'?" you might say. "They look great to me!" And they probably are great images, or move us in some way, otherwise we wouldn't have taken them. But one thing we often forget to do is look at the image from another perspective. The best way I have found to do that is to play with the RAW file.
Pick an image that you feel is top-notch. Open the RAW file. Now you may think I'm crazy, but trust me. This has worked wonders for me. Time to play with all of the settings without thinking about creating an eye-catching image.
Play with the white balance. Don't go for perfect, just see what the settings do to the image.
Now play with the contrast, brightness, shadows, saturation, etc. Mix'em up. Move one back to neutral and the other far left, one far right with another halfway somewhere, etc. Don't worry about if the image looks awful. Just see what the different combinations of settings do to the image.
Break out the cropping tool. Crop it square, vertical, horizontal. Adjust the cant of the cropping. Move it all around! A slightly different angle can make all the difference, such as in this image.
Now that you're done playing with everything, and combinations of settings, close the RAW file.
Now open it back up.
Look at the image. Are you seeing it differently?