1) Kemp is a centerfielder, and a great one at that. He has a cannon for an arm and has excellent range. Dunn is a below average leftfielder.
2) Kemp is five years younger than Dunn, meaning that he will be getting better while Dunn will be losing his skills. He also will be much cheaper than Dunn.
3) Kemp is a righthanded power hitter for the middle of the lineup, something that the Reds desperately need, especially with Bruce and Votto and possible Alonso coming up.
4) Kemp will provide similar run production to Dunn in GAPB. He's too young to really know what his ceiling is, but he is putting up solid numbers playing a pitchers park. He could easily hit 30 HR and be a 100 RBI, 100 runs guy in GAPB. The last two years, he has averaged around a .860 OPS. Dunn's career is .900. Have them switch ballparks, and they will have very similar OPS.
Basically, even if Dunn is better player, Kemp fills the Reds needs better, and makes them a better team. I don't want a team of the best players, I want the best team. Kemp instead of Dunn makes the Reds a better team.
A good example is when the Red Sox traded Nomar, their best and most popular player in 2004. The players they got back were not as good as him, but they made the Red Sox a better team. I think that worked out well for them.
By the way, the Dodgers would not trade Billingsly or Kershaw straight up for Dunn, let alone both of them and Kemp for Dunn. For that suggestion, I will call you crazy.