When I first started WD2, it seemed like a stepback. The RC car was annoying and the tone was too lax. After finishing the entire game, however, I can say with confidence that this was a great improvement over the original.
The biggest changes are with the characters, gameplay, and story. It is much more involving and interesting than its priori installment; over time you learn to love the RCcar and RCchopper, as they become integral tools for your method of play. There are also lots of non-lethal options that are very powerful, so it allows you to stay true to the lighter tone of the game. (I never really did like the idea of killing innocent cops or security guards).
While the original WD had a darker tone, the main character was much less interesting because he was too two-dimensional. Marcus is much more well fleshed out, and you feel for his allies, who are more or less equally well developed. The end result is a scenario where you are more invested in the story and your comrades.
The stun gun is a welcome addition, and gameplay is smooth (and its progression with upgrades is satisfying). SF is also beautifully done. All in all, I feel that this is just a very solid game and I recommend it