We currently have our Source Control settings set to enforce a Check-In Note called 'Reviewer' is set for every check-in in our projects.
However, for two of my developers, the Reviewer field is not showing up, and they can check in their changes without having to fill in the field!
I've cleared the Cache (as mentioned in many posts, this one in particular) on one of the developer's machines, but this did not correct the problem.
I've grabbed the "devenv /log" output from both developers and compared it to mine, but nothing jumps out at me there either - the versions of Team Foundation assemblies that both are loading into their IDE's are the same as mine, and any 'Power tools' that we load are also identical other than tools that we don't share at all.
We are using TFS 2013 Update 3, and Visual Studio 2012 Update 4, Visual Studio 2013 Update 4. The Developers on my team might typically switch between VS2012 and VS2013 at least once a week.