Expression indicators are a simple but good idea, especially when moving SSIS packages from development and test environments into production. You can already parameterize previously hard-coded values such as connection strings in the connection manager. This allows the server or environment in which the package is running to be detected by the package and the appropriate connection string used, without requiring the manual intervention of the DBA, with all the time and risk that entails.
However, up until SQL Server 2012, SSIS did not give an indication as to whether an expression was used within a connection. Now, you can determine it at a glance. Previously a connection manager that was expressionless would look like this:
Now, with an expression embedded in it (and that is for any property, not just a connection string), the same Connection Manager will change appearance to look like this:
Simple, but effective.