I remember the cards too, though I always thought that this sort of thing should have been built into the delta ticket envelope given to a passenger at check-in. (That way, one could simply tear off the card and hand it to the person--not have to remember to bring the things along on the trip, or have to find them once back at home.)

In any case, one can always send a quick e-mail regarding excellent service (using the Delta web site). These e-mails will be acknowlegded. Once, after sending an e-mail in recognition of a flight attendant's excellent service,I subsequently received a gracious thank you note from that same person.