When reading the docs of quite a few payment processors (for a simple cart check out) Mollie was quite hilarious. I kept thinking "this is it? This is all I need to do??" Then the masochist api's are also available.

And a serious lack of "dear customer, we are keeping all of your money for reasons we wont get into, screw you and your customers, you have no further questions." Which I consider a killer feature.