I think the most common, most serious and most in-depth mistake is number one.
No matter how many modules I've listened to, no matter how many demoparty tracking compos I've seen, in 99% the problem is with composition. The musician writes a good part and then just loops it. He can change the number of instruments, he can pitch the whole tune up half a tone or a tone, but in reality all there is is just one small part. You start the tune, listen it's first 20 seconds and you know that this is what the tune will be about and nothing will change.
The fact that most mod authors fail to write out a good theme development makes thousands of mods sound like sketches or unfinished raw material.
In fact, when I have started a melody compo, which was called One Voice Compo, I saw that most people failed to write a melody. They were used to wrapping their tunes into arrangements, sounds and rhythm, but they were not writing too much musical ideas.
So, I think this is the most important thing to watch out for. Make your tunes develop. Bring surprises. Let your listener get something he does not expect.