I went through the latest 2 pages in the music section, and honestly most of the critique I could offer would be almost be pasting the same thing in the different threads. Overall, we all have to remember some basic elements that contribute to interesting art of any form (painting, drama, music, writing, etc.)
1. Unity - quality of wholeness that is achieved through the effective use of the elements; tied together
2. Variety - quality or state of having different forms or types.
3. Balance - arranging the elements so that no one part of the work overpowers. (Symmetry, asymmetry)
4. Emphasis - Artist draws eyes or ears to one element.
For example I'll use Themiscyra by the PhonyBrony:
http://soundcloud.com/the-phony-brony/some-song-that-i-shall-name-in
Unity can be heard in elements. The theme playing through the song keeps bringing us back to the same idea. Also through the blipping synth that starts at 54 seconds helps tie sections together. The rythm is kept through the piece. Sweeps help keep the song flowing.
You can hear variety through the whole piece. He switches out the drum patterns, adds in sweeps, the blipping synth that floats over sections, at 2 minutes the whole piece almost takes a different feel, he adds and removes different elements and switches up synths. Variety is what I tend to stress most. It's annoying to go through the whole song and it's just the same thing copied and pasted 6 times over again. Looking at your favorite big time artists will probably show a good bit of variety. Variety keeps the listener interested in the music. A boring song might be structured very nicely, but if it's boring it'll go right on the top shelf.
Balance - he keeps sounds where they're supposed to be in the mix. Nothing is downright overpowering another element. One might have some prominence over another, but that's to keep the focus on the important idea of the piece. This ties into
Emphasis. Perhaps your emphasis is in the emotion of the melody, which may be more specifically an emotion you are trying to bring out of the listener. All the other elements support the main element. It shouldn't be hard to figure out what the main focus is, nor should it be a confusing wall of sound. Everything should be pointing toward and complementing the melody and making it as awesome as it can be.
Hope this might be able to help someone out!