in the build workflow for TFS 2010 I could drill down into Compile and Test, locate the Foreach Project loop and get the workflow variable assignment of BuildSettings.ProjectsToBuild (serverBuildProjectItem).
Is there alternative to this in the new streamlined build workflow for TFS 2013?
ProjectsToBuild is available as a workflow Argument.
A lot of things that used to be Workflow Variables (e.g. SourcesDirectory, BinariesDirectory, etc) have been moved to Environment Variables that you use the GetEnvironmentVariable activity to get their values. For the list of Environment Variables available that are set by TFS see here: http://msdn.microsoft.com/en-us/library/microsoft.teamfoundation.build.activities.extensions.wellknownenvironmentvariables.aspx