Gathering data is expensive. Gathered data is worthless.
Making data available is relatively cheap. Available data is valuable.
So: having data available on the web is good. Keeping your data safe in some sort of data-cemetary isn't that useful.
If you have to make a map, normally you'd break out your desktop GIS system. You'd get your own data and download external data and convert it. You'd do analysis and in the end make a map.
The alternative is to do everything on the web. Not everything that is possible on the desktop GIS is possible/feasible on the web. Analysis, for instance, is more limited on the web. But you don't need to convert/import external data as that's assumed to be available on the web.
Geonode (build with GeoServer, Django, and GeoExt) is a tool you can use for pure web-based GIS. You can upload and manage GIS data. They're big on metadata: you don't want just a dumping ground.
Maps are an important content type. Maps combine the data/layers and the styles.
Geonode allows many people to collaborate. For every user, you can see the data layers and the maps that he added. For every data layer, you can see the user and the maps where it is used. For every map you see the users that work on it and the data layers that were used. Those three items (map, data layer, user) are very helpful when searching for good data layers.
Annotations and comments help to get feedback on the data. Extremely helpful for instance during disasters when you can add a comment that a certain bridge has been washed out or a road is inaccessible.
Social network stuff is mostly not yet implemented, but they're planning it. (Ranking by number of downloads, by-user rating, and so).
Web mercator is used as a projection. Usable everywhere.
It uses open standards (OGC).
His conclusion: when the web mapping platform becomes part of the daily workflow, collaboration and participation aren't an extra effort anymore, it just happens. And… the data is available.