This statement gets thrown around a lot when discussing college, but I just don't see how it holds up. It is very rare for an undergraduate to do any significant research, so most of the learning comes from assignments and probably a little group work.
I'd argue that a big part of it is being given assignments that stretch you more than you've been stretched. You don't have to do original research to be geniuinely challenged and grow from the experience. You just have to be given an assignment that requires you to dig for answers and fail. You have to exhaust most of your options when trying to figure something out. It's something we should probably be doing much more to kids well before they get to college, but college seems to be where we start doing it, so that's where the value is.
Since this is slashdot, there will be a million posts by clever college students who are doing really well in their classes and see them all as a waste of time. "Nothing at a university can challenge me! I'm the hottest shit that ever was shat!" All I can say is that they either didn't choose programs that were challenging enough for their level of talent or they're unusually talented people--the most brilliant of the most brilliant--and the world was not really designed for them. Or they're badly overestimating their level of talent, but that almost never happens.