What will this bring? Well, the US govt has X agencies. The result of this decree will be that, within 12 months, all of the public will get to enjoy the thrills of having X incompatible web API's, one unique one per agency.
What data are you looking for that isn't available?
The only thing I can think of that's difficult to get is some law enforcement, military and "national security" related information, but APIs aren't going to change that.
I don't have a ton of experience with it (mostly USGS and NOAA), but the Department of Commerce and the Department of Interior both make a ton of data available.