Most users are familiar with how text fields work, and most text fields allow you to safely edit text without triggering something irreversible (like posting or sending). Changing fundamental behavior, like the return key, will cause the user to make mistakes, and those mistakes will cause the user to feel negativity towards your program.
That said, if the return key is one of many triggers that initiate a non-destructive action (like saving), then Mger's answer with the auto save indicator would be a very user friendly solution, as it provides clear feedback at a glance. But completely relying on the return key for an action is not intuitive.
While designing a text box where a user can enter a note, I want to eliminate the enter button and rely on the return key.
Why???