It has little to with percentage of good songs. Although that's part of it. A good album, in my mind, takes a concept, an idea, a time period, a place, and crystalizes it into music, into artistic expression. A good album is appreciated more in the whole than for its individual songs. A good album is one that you will discover something new in ten years after the first time you listen to it. It can have 2 songs or 20 songs, it doesn't matter. Look at Floyd's 'Wish You Were Here'; only five tracks, but it's one of the great rock records in existance.