From what I have read, SR4 is a superhero sandbox where you act out a comic fantasy in a world with a crazy amount of options and customization. There are a lot of throw backs to pop cultures sprinkled through the game. Kotaku should a brief clip going through 50 easter eggs in 5 min. The reviewer says its just a small taste of what the game has included.
Giantbomb did an hour long Quick Look. I watched it and will pick the game up when it gets cheaper. I'm in no rush. The take away was to buy it on PC, the PS3 and 360 versions are painfully laggy.