What characterizes a soup that holds well in a buffet setting?
I was really hoping that someone still in professional food service would answer this, but they haven't, so here's my list of rules based on memory from when I was:
Soups that are no good for holding:

Egg-drop soups and seafood soups are risky because of rapid spoilage
Miso soup settles out
Soups that are thickened with a butter or heavy cream emulsion; these tend to separate after being heated for hours
Pasta, barley and rice soups are problematic because the grains both absorb liquid while sitting and sink to the bottom.
Bread-thickened soups tend to turn to porrige
gazpacho actually does not do well; even though it is cold, after a few hours out it can ferment unless you keep it well-chilled

Soups that are good for holding:

Just about any pureed vegetable soup
Cheese-and-emulsifier thickened soups, like broccoli-cheese soup, do well
Bean soups with a moderate amount of beans do well (thick bean soups tend to solidify though), like vegetable/lentil, or brunswick stew
Chicken/noodle soup is a good standby if the noodles are very small and less than the chicken (see above)|
Thin dal, like a classic South Indian sambar, is designed for long sitting
Tomato/vegetable soup
&quot;Chowders&quot;, including corn chowder, hold well from a suspension/serving standpoint, but see above for the risks of holding seafood
Any clear broth, such as bone broth, consomme, pomegranate soup, etc.

That's all I have; hopefully someone with current buffet-running experience will speak up.