I had a lecture once on focus in arrangement.

The singing, as has already been stated, must dominate an arrangement. It is the primary focus.

The accompaniment should be secondary. It can overwhelm the melody by being:

1) Too fast
2) Too loud
3) Too high

At points in a song where the singer isn't singing, well the accompaniment can become primary focus.( Intros, Instrumental verses, Fills at end of lines etc)

There are plenty of exceptions to this but its a good starting point.