Giving players the tools needed to create custom missions in a game like inFamous 2 is a wonderful idea, as long as the tools are easy enough that anyone can use them. You be the judge.
Seems simple enough to me, but I still can't see myself having them time to craft an entire mission from scratch. It's the same issue I ran into with the LittleBigPlanet games. I was inspired enough to have plenty of lovely ideas, but lacked the time to implement any of them. Thankfully, inFamous 2 will let me download anyone else's creation and tweak that instead, saving me the trouble of having to lay a foundation for my exploits.
See inFAMOUS 2's Mission Designer in Action, More Details on Single Player [PlayStation Blog]