At Alfresco, we often refer to "the Dropbox problem". It doesn't look like anyone has written a clear explanation of what that means, so I will do the honors.
Dropbox is a very popular service for cloud-based file storage and sharing, coupled with a simple way of keeping files synchronized across all of a user's devices. I use Dropbox for keeping my personal notes and photos synchronized on my desktop, tablet, and phone. Dropbox works so well that it has become the standard for how a consumer oriented service should make a complex problem easy to solve.
But Dropbox's strength is also it's weakness. Dropbox is consumer oriented, and helps users manage the files that belong to them. It does not worry about whether the user actually owns the content that is being managed; the Terms of Service for the site requires the user to have ownership and the ability to grant redistribution rights to the company behind the service (a requirement which is often ignored). This simple model of ownership does not translate well to the enterprise world of team collaboration. Enterprises want to control the assets that are important to their work, because losing access to a critical document results in lost productivity.
Content that is produced on a team is not necessarily the property of any individual team member. If one person leaves the team, the rest of the team still needs access to the content that was previously managed by that member. Teams might not want any individual member to be able to redistribute the private documents of the team. And teams often want to search for a specific phrase among all the content produced by the team. The simplicity of Dropbox's individual-centric design does not support any of these common use cases.
"The Dropbox problem" is the name we give to the risks associated with using a consumer oriented storage service in an Enterprise setting. These risks are not unique to Dropbox, but shared by similar services such as Google Drive. Any service which an individual might use for hosting content independent of one's team is likely to raise similar concerns. When people move between organizations they often take the access to their content with them. Their previous team does not have control over the content and the other team members must recreate or do without the content that they collaborated in producing.
Even with this common problem, people continue to use these services to complete their professional work because they are so helpful. Careful team members will respect team policies towards redistributing content and will make sure to put content in a shared location before leaving the team. But these tasks are easy to overlook, and not all team members are careful. You are likely familiar with an experience like this one: the original copy of the contract can't be found, so someone tries to contact a former team member, who if they were following company procedure would have deleted all copies upon separating from the organization. This experience is getting more common in the post-PC era of Bring Your Own Device.
Enterprise content management attempts to give the organization control over the content that is being leveraged by their teams. Traditionally, ECM approaches have required a sacrifice of convenience for the user. But modern systems such as Alfresco attempt to address these problems with a minimum of hassle.
Though services such as Dropbox and Google Drive now offer team-based products which address many of these concerns, I don't think the Dropbox problem is going away. Unless an organization provides tools which preserve control while still being flexible and easy to use, workers will continue to use the free, individually-focused services to get their work done. It is the responsibility of information professionals like us to develop and deploy these tools.
Photo credit: jasonwoodhead23 @ flickr