I really don't understand why people get so worked up about this. If the FA is taking the time and making the effort to go around and thank pax, they obviously care about doing the best job that they can and the customer. This is so much better than the ATL based crews that just go through the motions, doing the bare minimum, as surly as can be. I always make sure to the let FA know that their hard work and effort is appreciated and does not go unnoticed.
With all the data that DL collects now, what goes around, comes around. FA's are human, and humans feel joy when they see that their actions have directly made someone else happy. Eventually, and perhaps even now, FA's are going to know beforehand whether its worth it to go out of their way for a customer. If the customer is going to be grateful, FA's will put in the effort for you, and if you're ungrateful, nobody is going to go out of their way for you.
I was in Y for the first time this year last week. The FA came around to check on me periodically to refill my glass and see if there was any additional snacks or drinks or anything else he could bring me. It wound up being one of my most enjoyable flights this year - I received more attentive service than the pax in F! I talked to him for a couple minutes, but did not inquire as to why I was receiving special treatment, but I am fairly certain it was because of the data they have collected on me over the years. Moral of the story...be nice
