(Part 2 of a series. See part 1.)
Other ways in which shape-note singing is an "alternative choral music":
Independence of parts. In most pieces within the shape-note tradition, the three or four voice parts are independent melodies. Each line is interesting by itself, and it may not be constrained by the other lines. For example, it's possible to have a a piece where each vocal part simultaneously reaches the top of its tessitura, or one where the parts cross freely. The resulting polyphonic texture has a constant fluctuation of intensity, even if the piece is sung at the same volume throughout.
In Supply Belcher's 1794 composition An Anthem of Praise (not originally a shape-note publication, but now found in the modern shape-note collection The Norumbega Harmony) there is a passage where the treble and tenor parts sing the same melody as a canon at the unison. It is marked "duetto" in Belcher's original score, and it may have been intended for two soloists.
This independence of lines is characteristic of European choral music of the 16th and 17th centuries. By the 19th century it was old-fashioned, which was why the European-trained American musicians of the era disdained it so much. But from the standpoint of today, there is much to disdain in 19th century choral music, which often sacrifices the integrity of the line to create rich vertical harmonies. While these harmonies can sound ravishing to the passive audience, it is alienating to the individual singer to become merely a speck of paint on a large canvas. And the only way to make such chordal writing interesting is for a conductor to impose changes in volume, much like an organist pumping the swell pedal (or, worse, the crescendo pedal) back and forth. This is very easy to overdo. Shape-note singing introduces us to a different aesthetic, at once more participatory and less sentimental.
The early American and shape-note repertories have their own delights, but they also can be a good stepping stone to the works of Tallis, Byrd, and Palestrina, for church choirs accustomed to singing chordal SATB music. Getting inside this repertory requires first unlearning a chordal, homophonic, expressive approach to part-writing, and then learning a linear, polyphonic, architectural approach.
Doubling of parts. In shape-note singing as practiced today, men and women sing both the tenor and treble parts, doubling the part at the octave as needed. In the Belcher example above, for example, a modern shape-note singing class would (ideally) have equally matched sections on each part, with each section containing both men and women. This is another practice that goes against common-practice European music because of its fundamental rule against parallel octaves.
But looked at another way, is it much different when an organist selects both 8' and 4' stops to accompany a hymn? The hymn melody is heard both at concert pitch and an octave higher, but the ear does not hear two different parts in parallel. Rather, the ear hears a single melody with more intensity and richness. Likewise, a vocal part doubled at the octave in a polyphonic texture can achieve this quality.
And from a practical standpoint, the ability to change octaves when necessary is a boon to the singer with a limited vocal range, who otherwise would not be able to participate in choral music.