From the aspect of phraseology: the lyrics should have different styles and requirements according to different purposes. For example, the Chinese style (ancient style) song a major point is that the language must be aesthetic mood, adding poetry elements is the best, followed by the requirements of rhyme; and street dance square dance lyrics more requirements of the word can not have a remote difficult to read words, must be popular and so on.
From the adaptation of tunes: lyrics to be sung, so good must be the content and structure are suitable for its corresponding tunes, for example, soothing music with deep lyrics, lyrics structure, long and short sentences can be used; and fast-paced music with a catchy, preferably with the structure of the lyrics and so on. (The tune of Piggy's Daughter-in-Law can't be paired with lyrics that don't have any emotion, can it? This example is for you to laugh, the reason is indeed so)