The simplest description I would give for inFamous is that it's a better execution of Crackdown's concept.  It has the freedom of movement, the territory takeover sub-game, and a fun arsenal.  What it doesn't have is the repetition that caused me to develop a lot of contempt for Crackdown.

Don't get me wrong, I did enjoy Crackdown when it first came out, but two-thirds of the way through the game, I felt insulted and gypped by the copy/paste missions.  The game offered a city that was fun to traverse, controls that made GTA look really bad, a killer art style, and collectables that I didn't actually hate looking for, but it is a shame that the developers couldn't come up with more than one type of mission.

Empire City is the most fun I've ever had getting around in an open-world game, excluding the 747s in GTA: San Andreas.  This game where the protagonist physically cannot drive a car managed to have a better locomotive system than any sandbox game I've played that did have vehicles.  Even when I was combing all three islands for the third time looking for blast shards, I still enjoyed just climbing around, jumping, gliding, and grinding block to block. 

 Speaking of those blast shards, every open world game that includes collectables needs to rip off the "ping" feature.  It's still just hard enough that you do have to work to find them all, but it makes it an obtainable goal.  Excluding Crackdown, I have never attempted to find those things in an open world game, and inFamous is the only one where I collected everything.

 inFamous had its stable of repeated quest types, but it mixed them up enough that it never bothered me.  The missions made sense in the context of the game, and the only type that ever seemed out of place was the "destroy the surveillance equipment" type.  They were still shallow affairs that seem typical of either superhero games or sandbox games, but sometimes that's what you want.  inFamous is well-designed, simple fun.