Some songs are short, maybe less than a minute. Some songs are long, some over ten minutes. A good song is one that plays on the radio once every now and then but in your heart continuously. It doesn't have to have music, or a chorus, or even words. If a song is felt inside of you, then it has fulfilled its duty. 
Some songs rhyme, others flow nicely. A good song is one that takes all of the tips for good song writing and ignores them, not as a sign of ignorance or disrespect, but as a sign of individuality. Too many songs sound alike, some words are the same, some of the music is similar, and that is what makes a song just music. But I'm talking about a song. One song. A good song. The way songs should be. If it can have an interpretation to one person that is completely different from another's, then it is no longer just music, but a song. 
Some songs make you want to smile, laugh, dance, sing, sleep, cry, or throw up. Some songs give you these feelings because of who sings it, who plays what instrument, when it was written, or in what situation you hear it. If you can feel all of these emotions from one song without any instruments being played, or without an angelic voice singing it, or without a particular situation in mind, then you have found not just music, but a song. 
Some songs are sung, some are spit, and some are whispered. Some songs are spoken quickly and some slowly. When songs are real music, you don't notice how it is vocalized, or how quick it is, you only care about drinking in every word, every syllable, that you can. That is a good song.