A good review will be one that is constructive and helpful to the artist. Giving the artist constructive comments like, "the bass line was too far in the mix" or "the vocals were too quiet in the middle" can help point them to some potential weak spots in their song. Also, giving comments like "the melody line was very catchy" or "great guitar work" will let the artist know what they are doing right. Stick to the golden rule - review others as you would like to be reviewed.
And also keep in mind that it is not helpful to the artist to read comments like "I don't care for this style of music" or "This song needs work." If you do not care for the song, feel free to tell the artist, but let them know why and suggest some ideas on how the song could be improved.
Please don't allow your comments to become personal attacks on the artist or on the genre of music they play. The Broadjam peer review mechanism is meant to help everyone in the community, and being insulting will only decrease your chances of receiving quality reviews in return.