The 5 of Cups represents to me someone who tends to carry a lot of guilt, who leans toward depression, who analyzes the mistakes of the past rather than looking toward the future... Even when the person's blessings- those full cups- are pointed out to the person, this ends up being just another source of guilt- and the person is then hard on himself (or herself) for not being thankful enough...
Now of course this card can represent a minor and passing regret- no big deal in the grand scheme of things- but if this card comes up as representing a core quality of the person, then these initial descriptions might apply.
I know we each have our own "takes" on the cards, and we each have our own experiences that give layers of meaning to each card, but for what it's worth, the 5 of cups is a card that absolutely represents (for me) the main qualities of my father who has battled depression his entire life. He is so beautiful, has so much to offer, and he does have many blessings (including his incredible talent as a classical pianist), but he lives in the past and wonders what he should have done, could have done in every situation going all the way back to his childhood (and he is in his mid-60s now)... and he just about drowns himself in regret. I see the man in the picture (of the RWS deck) as an emotional person, someone who is mourning... but I feel some of the swords energy in this card as well. Emotions are often fueled by thoughts; if you focus too much on analyzing every past event from every possible angle, you really can drive yourself crazy, become depressed, and you're much less likely to see let alone truly appreciate the blessings that come your way. I'm typing in a stream of consciousness right now, but I hope this still makes sense...
"The opposite of a correct statement is a false statement. The opposite of a profound truth may well be another profound truth." - Niels Bohr