I am a big fan of leaving feedback, it is the only way to improve. Over the years I have left plenty of cast compliments and on occasion reported bad behaviour by CMs. When it is 8-10 out of 10 or 4 or lower.
I wouldn't do it for bland behaviour. If someone has just passed because they looked the part and were friendly, it is just something I tend to forget over time. Which of course doesn't improve the service/product.
For example, on my last cruise, my m&g with Moana was disappointing. And that was because all she could talk about during the interactions with myself and my friend was Maui. Moana is an interesting and well-written character in her own right, she doesn’t have to talk about the man in her story constantly. Was it a bad m&g, no. Was it good, no. I was just glad that the lines on a cruise are shorter than in the parks and that I had met her, because Moana was missing in my list of characters to meet.
I had some feedback about things going wrong on this cruise, I have sent to
DCL, but I left out Moana. Mainly because when i tried to put it down to paper, I felt like I was nitpicking. And I didn't think it would help.