How would I define a “traditional hymn” in contrast to a “contemporary worship song,”? Well, here are a few points to consider.
The difference has nothing to do with when the song was written, which is why I actually don’t like “traditional” or “contemporary” as modifiers. I prefer to call what I’m describing here “classic hymns.”
First, a classic hymn is strophic, that is, it has multiple stanzas with the same melody. This aids congregational singing rather than being a performance. Refrains are not common in classic hymns, but they do exist. Also, a classic hymn does not have a “bridge” or other pop conventions.
Second, the lyrical content of a classic hymn is doctrinal and theocentric. Even more, subjective texts are more directed toward God or about God rather than self-oriented.
“Contemporary hymns” could rarely fit the two previous characteristics, but they fail the following.
Third, the tunes of classic hymns are objectively singable. The vocal range of the melody is typically around an octave, where the melody mostly stays and not too high or low. Again, this makes the hymn congregationally singable rather than a performance. Even “contemporary hymns” have huge ranges that are not objectively accessible. Singability is a huge key difference and it can also greatly affect the unity, or disunity, of a congregation.
Fourth, rhythmically, the tunes of classic hymns are simple. I think this is one of the key differences between classic and contemporary tunes, and again this aids singability. The interest in a classic hymn tune lies in the melody and (if present) harmony. Classic hymns have interesting and singable melodies and simple rhythm with a lot of interest in the harmony. Contemporary songs have boring, static melodies and practically no harmonic development at all, but an emphasis on the rhythm and where they (most christian bands and songwriters) have to create “interest” is in very complicated rhythm. The problem is that people can’t sing it. People can sing simple melodies with simple rhythm, and then the interest is in the harmony. But if the only interest is in complicated rhythm, they’re not singable unless you already know it (from radio). I think this is an essential difference between classic hymn and contemporary worship songs.
Finally, classic hymns can be sung without technology or even instrumentation. Contemporary songs require them.