This is sort of related to @IO+'s profile post, but it ended up being a little off-topic:
I think a good vocal synth song comes from at least two areas. First (though possibly less-importantly), that the lyrics are written well, and in a way that both clearly and artistically attempts to convey ideas that are important to the producer. Second, that the music be good--that it embodies the appropriate mood, has an appropriate energy flow that shifts and moves through the different parts of the song, and has both constant elements that hold the song together and elements that vary and make the song change pleasingly over time.
I think a good vocal synth song comes from at least two areas. First (though possibly less-importantly), that the lyrics are written well, and in a way that both clearly and artistically attempts to convey ideas that are important to the producer. Second, that the music be good--that it embodies the appropriate mood, has an appropriate energy flow that shifts and moves through the different parts of the song, and has both constant elements that hold the song together and elements that vary and make the song change pleasingly over time.