From an ad in The London Lancet of 1886. (Later just called "The Lancet")
"Seven cardinal virtues should be found in a pen. It must be elastic, well tempered, durable, even-pointed, easy writing, well shaped and neatly finished. Esterbrook's have all these qualities in perfection."
"Elastic" means that it's not a nail, there's some give to the pen. That give can be from flexible tines, or a spring in the body of the pen. It's not a measure of "flex" (spread of tines) as we think of it today. It is the opposite of a "nail."
Well tempered: will spring back well from any flex, will not bend without coming immediately back into shape
Even-pointed: Tines are aligned and without flat spots or roughness. Steel pens of quality manufacturing need no adjustment or smoothing.
Easy-writing: smooth tip that allows you to write quickly and easily without over-worry of a too-sharp tip. Many nibs favored by calligraphers have very sharp tips and so while they can produce extremely fine hairlines, they are slow and require great control to write with. A good every-day pen should write easily
Well-shaped: I guess this just means that it looks good and is a comfortable size? Some of the novelty shapes popular with European makers were never really sold here. I've never seen an American manufacturer make any pen in the shape of something else with one exception. Esterbrook, in their very earliest years, did make a version of the pointing finger pen. But they are extremely rare (only two examples are known) and weren't made after 1883.
Neatly finished: an even color and smooth surface are two signs of a good finish.
If you're interested in how steel pens were made to get these characteristics, I have a description on my site.
https://thesteelpen.com/
