Love

Of course punctuation is extremely important to convey the meaning of a sentence. You have given good examples of it with these sentences. The following is what I think these sentences might mean:
"What is this thing called love?" - You are asking or wondering about an explanation of the concept of love.
"What is this thing called, love?" - You are addressing someone by calling him or her 'love', and asking for the name of a thing. Like if you ask "What is this, dear?"
"What? Is this thing called love?" - Here, you are being incredulous about calling a certain feeling 'love'.
"What is this thing called? Love?" - You are asking for clarification on the concept of 'love'. This is implied by the second question "Love?"
"What is this thing called? 'Love.'" - 'Love' is the answer (this looks like a song) to the question "What is this thing called?"
I'm going to write down your examples, maj. I think they are very good to explain the difference in writing a comma or not. By the way, a comma is really a pause made in speech. This may come in useful when reading these sentences aloud.

(h)