So I will answer all of these at the same time with the same answer. The OP was asking what makes DCL worth the extra money. As I have been careful to note, IN MY EXPERIENCE I have had consistently better service on DCL than the other lines (and this is backed up a little by some inside knowledge of how the lines work).
EVERY PERSON HAS UNIQUE EXPERIENCES. First off, every cruise is different, each people have different staff. The ships behave differently. Add on to that, if you take 2 random people and give them the same experience they will have completely different memories and viewpoints about it. I have seen two people in identical situations where one had a wonderful time and another had a terrible time.
Saying that I have personally had consistently better service on DCL does not mean that the random times you have been on it were perfect or that the random times I have taken RCL or Carnival are proof positive that they have lower standards.
I will tell you that I will always put my personal experience above random people's experiences on a board.
So to answer the OP AGAIN and more clearly, IN MY EXPERIENCE, I have found DCL to have consistently better service than I have had on other lines. For that reason, I am more inclined to spend extra on DCL knowing that I am more likely to have great service and my confidence in the brand that if things are not right, they will bend over backwards to fix it while I do not have confidence that my experience will be as good on other lines or that they will take reasonable corrective action if there is a problem.
To that end, I really only sail other lines now if I am interested in a destination that DCL doesn't have.
The people you quoted while I don't know them personally I feel I know their cruising habits and opinions from seeing them on here over the years. Them and others I value their opinions and is why I still lurk and post on this site once in a while. I am getting to know some over on CC too now on both the RCCL and Celebrity boards. So yes the random posts on here I read carefully, but there are quite a few peoples opinions on here I do trust. I never try to make my opinion as the one everyone should listen to, I just state my observations I have had over 16 cruises. Without encouragement from others on this site I might not have set foot on my first RCCL cruise.