All data portals and APIs provided by the government organizations (or NGOs) have one problem - data quality. And if data is incomplete, stale or inaccurate, it is useless and sometimes even harmful. Every developer/researcher using such data starts anew, cleaning and enhancing the data. And all those efforts provide no benefits to others. We participated in a dozen of such projects, with city, state, federal and non-profit data. I believe the solution is to open the data for anyone to edit, conceptually like Wikipedia, but going one step further, giving equal access to both people and apps.
I know that it immediately raises a ton of concerns, but if the system is put in place to track all modifications, and all modifications are based on a reputation system, a-la http://stackoverflow.com, then it will be a win win for everyone. We have created such a system and pitched it to the NYC government. Unfortunately they were still so excited that they provided so much of "their" data to the public, that they could not see that all previous data sharing efforts made no real impact. This is the slide deck that we presented to them.
We created a set of concept apps for different branches of the NYC city government that illustrate how this Mutual Data model might work. You can see some of those apps on http://urbien.com
In the last year we have evolved the model into an open source platform http://github.com/urbien/urbini and evolved the idea of shared/mutual data into a shared data graph that enables apps to form an appnet. See readme on our Github repository for more info.
The idea of a shared Data Graph can be considered a next step after a Facebook Graph, which had a huge impact on people's lives and in creating a new app ecosystem.