"This project is incompatible with the current version of Visual Studio"

VS 2012 has different project type support based on what you install at setup time and which edition you have. Certain options are available, e.g. web development tools, database development tools, etc. So if you're trying to open a web project but the web development tools weren't installed, it complains with this message.

This can happen if you create the project on another machine and try to open it on a new one. I figured it out trying to open an MVC project after I accidentally uninstalled the web tools.

If the message

This project is incompatible with the current version of Visual Studio

is due to an attempt to open a project targeting .Net 4.5, then the "solution" or workaround is to edit the .csproj file and change the TargetFrameworkVersion from "v4.5" to "v4.0". That at least allows the project to be loaded, although it may result in compiler errors if the program is dependent on 4.5 features.

I just got the same error message with a couple projects after installing Visual Studio 2015 Update 3. For me, the solution was to install .NET Core