To me, simplicity is the biggest factor in the quality of a song, because if it's over the head of the average listener it's doomed to failure. Now, you could argue that the listening public as a whole lacks taste and is under-educated when it comes to appreciating music of a cerebral nature, but I think the key lies in disguising that complexity. Beyond the general philosophy, however, a great song has a magnetic intro, and ideally a powerful verse, chorus, bridge, ending, etc. My advice to aspiring composers is to learn great tunes from great writers. Strength generates strength!~QM