Is there a difference between ‘where is the catch’ and ‘what is the catch’ other than the latter seems to be more popular?
Many thanks,
Torsten[YSaerTTEW443543]
TOEIC listening, question-response: Are you sure you won’t have an appetizer?[YSaerTTEW443543]
I vote no, Torsten.
The essential meaning is the same, but the mental image they evoke is different.
“What is the catch?” basically means, “Tell me the thing that makes the deal less desirable.” Mentally, I see a person pointing to something.
“Where is the catch?” often means something like, “Okay, I’ve heard your whole story, and I can’t find the catch in it, so please unwrap it and show me where it is.” In my mind I see a person unraveling something and pointing to something inside it.
IMO,
What is the catch? = I don’t know what the catch is.
Where is the catch? = I can’t find (out) what the catch is.
Well put, Haihao. 
Hi Torsten,
This could well refer to a financial deal where the potential customer is supposed to be able to make a huge profit. You contribute £1,000 and you stand to make £10,000. Your initial reaction could be: What’s the catch? This suggests that you are suspicious about how this will work. You are told that there’s is nothing underhand about the deal and it is perfectly straight forward. Your next question is: Where’s the catch? This suggests you are not convinced, you are still suspicious and you want to know exactly where the weakness of the deal lies.
Alan