My guess is, because the CR is one of the least-themed resorts, it also comes across as one of the most-"common" resorts. Once you're outside the lobby or cocourse and in your room, nothing, except maybe the view, screams "Disney" at you. Consider the theming of AK or WL. Both are Deluxe Resorts with standard rooms costing less than standard rooms at the CR, yet those rooms are well-appointed with little bells and whistles that comes across like a genuine effort to make you feel somewhere special. I find the same to be true even in most of the moderate resorts (except for maybe Coronado Springs). Whereas the CR rooms, while quite nice, still come across as more functional and practical than magical. But you're still paying quite a bit to stay there. I think that's where customer disappointment comes in-paying Disney prices to stay in a Disney room that doesn't "feel" particularly Disneyish.
I would further suggest (so CR-lovers don't think I'm a big jerk) that the people who tend to enjoy the CR are usually either
a: People who don't need to feel "Disney magic" to enjoy a nice hotel room. Perhaps that intabgible feeling we all get when we get to WDW is lost on them, so having a room with less-blatant theming is welcome to them...
OR...
b: People who come to the resort with so much Disney magic of their own, they don't need the accoutrements of a well-appointed and themed room to be excited about being in the world. For people like that, to be so close to the Magic Kingdom, to see the monorail enter and exit the A-frame, to be able to see fireworks & the water pageant outside their hotel, perhaps outside their window, more than makes up for an unthemed room.