There is a syntax change for the parameters in 5.1. Any usage of parameters with $ prefix now require encircling the path in parentheses. For example, $FileSystem1.FullPath in 5.0 should be changed to $(FileSystem1.FullPath) in 5.1.
When using output from dataflow in a decision expression, the syntax has changed. You now have to say
RunDataflowTask.Output.CompletedSuccessfully instead of RunDataflowTask.CompletedSuccessfully where RunDataflowTask is the name of the run dataflow action in the workflow.