Would it be possible to add a warning regarding String.Concat in .NET 4.5?
There is an issue reported on Microsoft's Connect website: https://connect.microsoft.com/VisualStudio/feedback/details/780770/string-concat-slow-in-net-4-5#details
Looking at the results, it appears that String.Concat is up to 200 times slower than similar operations. Microsoft have given a workaround, but not a solution.
Given the large difference in the results (if it were only a few ms I wouldn't ask), I think it would be important to tell people that potentially they could kill the performance of their application.