Guilty Pleasure, focused-attention, Indeed!
Fun factors for me:
1) Seeing your favorite Nintendo characters play well on the tennis court is semi-nostalgic;
2) I simply love the sport of Tennis and playing with awfully cute characters take my mind off its serious, edgy side! All in all, the "stripped-down" tennis game works simple and fine!!!
Finally, I give this game title 5 out of 5 Mario stars or simply an 8 out of 10 because despite Gamespot's poor 5.5 Overall review, I already purchased it and bottomline is, it's not what you ain't got. It's loving what you already have!