These are all good stories with excellent turnouts. I admit I expected to hear some horror stories and real complaining, but I haven't really seen any.
The key certainly does appear to be following up with a rational letter explaining what was wrong. Most people say they also noted what was right about the cruise. That indicates a key component is to show them you plan to stay with the cruise line, which makes it worthwhile for them to keep you as a customer.
In addition - it means you are likely to take what appears to be their favorite form of compensationh - a future cruise credit.
It is the people who say "future cruise credit - no way am I sailing on this line again" that end up with smaller (or no) compensation.