In my travels and implementation paths for implementing Team Foundation Server and making it coexist or migrate from Visual Source Safe, there have been some significant challenges in educating clients about the change, specifically developers who are used to the seeming simplicity of VSS.
The idea here is that TFS is different and now behaves like a true enterprise source control system, as opposed to VSS which is targeted towards small (really small) implementation teams. Once the differences and benefits are explained, developers are eager to migrate to TFS. Here are the core differences as outlined by MSDN. Definately give this a read if you are championing TFS in your organization.
The biggest challenge for me? Unlike VSS, TFS does not get latest version on checkout automatically. It violates the workspace island concept that TFS (and other source control systems) have.
Also, here is a great article and walkthrough on now to migrate your VSS database to TFS (in order to preserve all versions, historiy, labels, etc.) using the VSSConverter comand line utility.