To learn what it is, however--besides the obvious category of "praise" that one would infer from all of those Halleluiahs (praise y'all the Lord) in the Psalms and elsewhere--let's take a look at Ephesians 5:19.
addressing one another in psalms and hymns and spiritual songs, singing and making melody to the Lord with your heart, 20 giving thanks always and for everything to God the Father in the name of our Lord Jesus Christ
Notice here that we are to address, or speak to, one another in three (?) types of music. OK, so we're not bound to just the Psalms, and whatever we do, we "speak" to one another. Some kind of information is being imparted, and hence I would affirm that the song ought to (a) contain some theological information and (b) ought to convey it clearly--no coffee shop mumbling or heavy metal screaming a la
We can infer from the second phrase of Ephesians 5:19 that believers ought to join in the singing, and that the melody ought to be somehow in our hearts. Hopefully this is not too much of a stretch, but a "melody in our hearts" can imply both that the Scripture resonates in our hearts, and that the way the song is formulated is winsome--it is poetically and musically good. It ought to have some discerning marks in meter, rhyme, alliteration and the like, it ought to have a decent tune (no amelodic hymns, please), and the presentation of the song (hymn, Psalm) ought to be appropriate and memorable.
What seems very clear is that James MacDonald's prescriptions for music are pretty much dead wrong. It should convey theological content, is not as a rule repetitive or simple, there is no restriction on the grammatical person therein, and in light of the range of topics presented in Psalms, it doesn't as a rule need to lend it self to physical movement. Imagine, for example, trying to dance to "When I Survey the Wondrous Cross." It is, like many of the Psalms and Lamentations on the fall of Jerusalem, solemn.
Or, to address the final part of MacDonald's "Vertical Church" prescriptions for music, "When I Survey" builds its emotional value off the horror and awe that we ought to see when we consider ourselves in the shadow of the Cross--and does not need to be "emotive" in its wording because it is already powerful in its content.
In short, I would argue that those who would write, or perform, music in the church can do little better than to--beyond learning the Scriptures and possibly even hearing or reciting the Psalms in the original Hebrew--learn the depth and breadth of good music and poetry, including secular sources. Read Ben Johnson, the Bard, Frost, and others to get a "feel" for powerful poetry. Listen to a variety of music to get a "feel" for powerful music--concentrate especially on the music which is notable enough to be remembered on the "oldies" stations and such. You may quickly see what dreck is being pushed on us from many of the sources you may be hearing at church and elsewhere.