MBP 15" Retina problems: one display hinge is way too tight
And the final cause: both hinges are fractured, and in the same place. Here's the right hinge, unscrewed and levered open (and after lubrication; see below):



.. and the left hinge:



Each hinge consists of a tube (anchored to the display, and with a cable down the core), plus a bracket (anchored to the main chassis). The bracket consists of two curled pieces which wrap around the tube in opposite directions. On both hinges, the inside "curl" is broken at the base of the bracket:



This explains another symptom; where the display used to open out to a certain point and then hit a stop, it now opens to almost completely parallel to the table.

Now, I'll be the first to admit I've not been gentle with this computer (they're tools, not shrines to the Great God Jobs), but it's still surprising that both fractured in the same place.

The solution: as you can tell from the pictures, I lubricated both hinges with lithium grease (being as moderate as possible). The display now swings fairly freely, so the previous tension shouldn't cause any more damage. I may have to rig up some sort of brace to keep the display from falling flat onto the desk, though...

Edit: And yes: the hinges are now too loose, and the display will fall open or shut if it's more than ten or twenty degrees from vertical. I now have to prop the display up with my knees or a book. But, this is still better; I no longer risk breaking the chassis or the display frame. I'm thinking about portable ways to hold the display at a given angle; the final solution would still be to replace the hinges (if I can get the parts).