Not being an expert in hymnodic history, I'll keep my comments generally about my own perceptions and experience. Typical characteristics of Hymns: strophic (one tune for many verses), designed for four-part choral singing, syllabic text-setting (each syllable gets its own note) with mostly simple rhythms, linguistically and theologically complex and developed (i.e., lots of words, poetically arranged with carefully developed theological messages).
Like many living musical traditions, the Hymn tradition has shown the remarkable ability to incorporate different musical traditions throughout its history: ancient folk tunes ("Let all things now living"), renaissance dance tunes ("A Mighty Fortress is our God"), Bar songs ("We praise the O God"), Gospel music ("Pass me Not O Gentle Savior"), and even elements of contemporary popular music (such as in "Here I am, Lord" or "Gather us in"). Yet hymns have kept most of their trademark identifiers (listed in the previous paragraphs); these other musical influence have not replaced hymnodic stylings, but have been adapted to fit them. Most importantly, new hymns continue to be written, and the best name for these would be "contemporary hymns." It's what makes hymnody a living tradition.
Rock/Popular music, its roots largely coming from African-American rhythms and blues harmonies, also has its own set of typical music characteristics and stylings. The kind of pop-based music most churches sing typically feature: syncopated rhythms, melodies with extensive use of repetition (an important musical device, and what makes the tunes "catchy"), shorter stanzas that are linguistically closer to today's vernacular speech. These songs are less rigid with metrical consistency--each verse may have a different number of syllables in each phrase, and each phrase has a different number of syllables, too. That is, many hymns can be categorized by a number meter (e.g. 188.8.131.52 D), while many pop-based worship songs cannot (marked "Irregular").
Mark Allan Powell has recently compiled an Encyclopedia of Contemporary Christian Music and was interviewed about it by Christianity Today. He makes some important comments about Christian Pop/Rock:
- It is a musical tradition, and deserves as much attention as other kinds of church music.
- The people who write it are sincere, faith-inspired, and incredibly diverse.
- It's purpose is to express and embody Christian piety, in contrast to the hymnodic tradition's usual practice of expressing Christian theology. To quote the article:
Two aspects of faith are theology, which can be explained as matters of the head, and piety, which is matters of the heart. I usually call this the prose and poetry of faith. Theology is the prose, and we need good theology to know what it is we believe and know how to articulate what we believe.
Piety is the poetry of faith. In it, we pay less attention to precision than to honest expression. Contemporary Christian music needs to be theologically sound, but its real strength is in the realm of piety. It touches the heart, it's relational, it's empathetic and it's emotional in a way that is completely appropriate for a holistic understanding of faith.
What's helpful about understanding these things is that it allows us to appreciate each musical traditions for their respective strengths, removing us from judging church music solely on personal preference. It helps to free us from self-centeredness and toward communal identity, away from isolation and toward community.