In Zinsser's "On Writing Well," he says to avoid the exclamation point altogether. I agree it's juvenile and robs the reader of their chance to feel the impact of something for themselves.
However, I've run into a use case that relies on exclamation:
In writing about cognitive science, I'm often citing studies whose conclusions counter common sense. I'll explain the study and then write the counter-intuitive outcome:
"If, however, they're told xyz, they're less motivated!"
(In this case, for instance, you'd expect that the subject, when told xyz, would be more motivated.)
The only other way I can see to accomplish this is to say, "they're less motivated, not more"––but this seems dull. If I don't explicitly point out that this is a counter-intuitive conclusion, I feel the reader could easily miss it. That's why I've fallen back on the exclamation as an aid for the reader to easily spot counter-intuitive findings from the research.
Are there other ways to mark the unexpected or counter-intuitive, other than the two I mentioned? Is the exclamation forgivable in this case?