a good song is ...

familiar and fresh
Firstly, the song connects with you as a singer in some fundamental way. The words strike some chord within you. The style of the song is not unfamiliar, so that even if the song is new, you can still relate to it. Secondly, the song offers something new. It is not merely a clone - copy, replicate - of some other song. Instead, it reflects a dimension of truth and experience that is unique to the song and makes it worth going back to again and again.
speaks for you and to you
Firstly, the song expresses what is already in your heart. Secondly, it counsels you in some way, drawing your attention to spiritual truths that increase your faith and devotion.
has a strong identity without being flashy
Firstly, the song's title, melody line and message are clearly focussed. It is also clear how the song might be incorporated into public worship. The song clearly draws attention to God, though, and not to itself.