I'm currently buried underneath 1,000 research papers to grade (okay, so maybe that's a slight exaggeration, lol). Still, I wanted to come up for air and offer a quote that one of my students posted recently.
Before I get there -- we all know that too much of a good thing is...well...too much. I always give the example to my students of "Good Will Hunting." Sure, Oscar-worthy movie. Sure, incredible performances. But honestly, the first time I saw it, I lost track of how many times the "f" word was used. Yes, it enhanced characterization. It made the dialogue feel more genuine, more realistic. And, it made the contrast between Will and the new circle of friends even stronger. I get it. I understand its purpose. But sheesh - did it have to be inserted in every other word? I'm no prude, but after awhile, it became a distraction. It started to take away from all that Oscar-worthy stuff (much like any word said over and over and over again, 50 times in a row, will quickly lose all meaning).
I tend to think less is more. For instance, imagine a meek, sweet, God-fearing, never-cussing grandmother shouting a profanity when she stubs her toe, then covering up her mouth in shame (even though she's alone). Fully unexpected, and fully powerful. That profanity, as opposed to Will Hunting's, packs even much more punch because it's used SPARINGLY.
Another quick example would be a book that hits the reader over the head with SYM-BOL-ISM. It punctuates each-and-every-scene with the deeper theme, that nearly chokes the reader with its earnestness. It's too much. It's over the top. Let the readers figure it out for themselves. Use words and symbols SPARINGLY. Otherwise, they lose their value, which is the opposite of what we're hoping to achieve.
This, I think, is what Mr. Twain means here (and he says it so much better than I):
God only exhibits his thunder and lightning at intervals, and so they always command attention. These are God's adjectives. [If] you 'thunder and lightning' too much, the reader ceases to get under the bed, by and by. ~Mark Twain