Wikimedia Commons is a very important resource, but it also has policies which many people will find difficult to understand. They cannot just upload whatever they want, they have to provide full source information, and they should have at least a basic understanding of licensing.
Many ideas have been mentioned here to somehow limit uploads by first timers. One that I haven't heard mentioned is to require every new user to go through some interactive tutorial process that explains the basics. How could this be done?
1) Create a new permission for uploading. 2) Any user who doesn't have the permission, and tries to upload, is presented with the tutorial. 3) Once they have completed the tutorial, they receive the permission.
On the implementation level, it seems that this can be done as an extension which a) hooks into the upload process and checks whether a user has the permission, b) displays a set of pages from the MediaWiki: namespace (important so the tutorial can be localized using the "/de", "/en" .. subpage syntax), each of them with a "Previous"/"Next" button. Only when a user has viewed the final page in the sequence, they would be given the upload permission.
All existing users (except sysops) would have to go through the process as well.
Future refinements could include interactive questions/answers about copyright issues. As for bots, bot status has to be set by bureaucrats anyway, so these could also give bots the upload permission.
Does this idea make sense? If it works for Commons, a simplified version might even be useful for each individual Wikimedia project.
Erik