Was that easy! and
Am I hungry! are particularly American constructions. I don't think we have this inversion for an exclamation in BrE.
When we use the negative, it's formally an interrogative, so if I say
Isn't he clever? it's like asking
Don't you think he's clever? However, we often put an exclamation mark here too, because it's not a real question that somebody has to answer.