The title will probably make some people argue that it was never gone on the first place. That's true. But there are some resources out there trying to convince you that User and Power User roles are basically the same thing. Some even state that Power User is there for legacy reasons but it's not and will not be used by Liferay in a future. While I'll refrain from making any statements as to what Liferay will do in the future, I think you shold know that currently (which is Liferay version 6.1) User and Power User roles provide different permissions set!
The confusion comes partly from the fact that by default all users are assigned to both roles. Thus from user perspective it would theoretically make no difference in which role given permission is set. But keep in mind that users can be removed from Power User role at any time. So before you do set a permission please think whether it should apply to all users ever (then add it to the User role) or it should only apply to given group of users (if so add it to a specific role - for example Power User).
To add to the confusion, in 6.1 a new button called "Clean Up Permissions" showed up in Control Panel. While the description tries its best to describe what it does, it may not be very clear if you believed that User and Power User are the same. To help you better understand, I generated the following table which shows what are the default permissions for Guest, User and Power User roles before and after you click on "Clean Up Permissions" (click on the screenshot below to download the PDF):