What does "tell us what you really think" mean? It seems to be used when someone is telling us what they really think...
The phrase is often used ironically as a reaction to a strongly-expressed opinion.
A non-ironic use, to draw out a stronger opinion:
"So, what do you think of the website's new layout?"
"Oh, it's okay..."
"Come on, even I know there are a few mistakes in there; tell us what you really think."
An ironic use, as a reaction to a strong opinion:
"I hate him, I never want to see him again, and I hope he catches a terrible disease!"
"But tell us what you really think about him, Sally."