The gameplay in inFamous is excellent. This game takes a sandbox--an open ended game in any new level. Basically, you will play in a third person perspective, while walking around parts of the ruined city, and you are allowed to go anywhere. Of course, on the way, you will be fighting some enemies that will be no problem for your lightning power. Continuing missions and exploring specific parts of the game will net you more exploration--at the end of the game. And, you will have a total of three islands to explore, all the while you can do anything you want to do.
Think of this game as "Spider-Man" meets "God of War". Or more simply, God of War as a sandbox game. If any of you played the movie Spider-Man game, it has always been praised for one main thing--it's degree of open world and exploring. While the God of War series is famous for the intense graphics and bizzare superpower that the hero Kratos has.
This game just combined both of them and created them into one good game. There are endless possibilities here, and it's up to you to discover it.