One of the main ideas of the essay was the idea that tragedy affects people, and not gods or higher beings because they are not relate-able to the average person and real people experience tragedy, and one of the quotes that reflects this idea from the essay for me would have to be, "A tragic writer does not have to believe in God, but he must believe in man", which makes sense because they're not related to tragedy.
The essay also touches on the idea that while tragedies have lots of death, that doesn't necessarily make them sad. Krutch uses Romeo and Juliet as an example. While both characters died, there was a bittersweet part of it because their death caused some good, whether it be that they were now forever bound together in death or that it helped patch things up between the Capulet's and the Montague's. So tragedy could in fact show that death is rather bittersweet.