Elite US schools have an unparalleled reputation built upon hundreds of years of pedigree, and as such, there is incredibly fierce competition for entrance. Once you are admitted to an Ivy, the hard part is behind you, and you can get on with your courses. Uwaterloo is the the reverse: it is almost trivial to gain entrance, but surviving the devastating curriculum is another matter entirely.
Elite US schools have the best teachers in the world in my opinion, as the quality of their online lectures have shown. However, it appears that the assessment difficulty is similar to a top level Canadian schools (UofT, McGill, etc.). This is the crucial difference between UWaterloo and every other university.
UWaterloo assessment is brutal. In every way that an assessment can be made hard, it is done so. The tests are purely theoretical, making rote memorization useless. Furthermore, they are very long, putting slow and deep thinkers at a disadvantage. On top of all this, there is an extremely strict system of producing completely new exams and assignments, while making sure previous years' materials are not released. Combine this with an uncompromising curriculum in terms of breadth and depth, and you have one of the most rigorous CS programs in the world.
Now you can argue whether all of this is unnecessary (whiteboard vs terminal coding), but the truth is, the sheer difficulty of the challenge tends to weed out the uncommitted. The reason why UWaterloo grads are humble, is because they have had their egos beaten out of them. Without exception, every UW Math/CS undergrad is one of the smartest people in their high school/town/province. They come to UW, and within the first year, they have a serious confidence crisis as they are failing courses and getting marks they've never seen before (50s 60s 70s).
Once this occurs, they either quit, or swallow their ego and just focus on learning the material and preparing for the exams. There is no psychological foundation for arrogance, as even the best and brightest suffer failures repeatedly.
To summarize: UWaterloo is war. Your morale suffers, your friends fail and leave, and you lose all hope. But the few who survive are battle-hardend engineers by the end.
Unofficial UW motto: "Cry in the dojo, laugh in the battlefield."
Addendum: Co-op terms are a refreshing break from this academic war zone. I was always amazed at how simple, straightforward and fun actual work was in comparison to the ruthlessness of the curriculum.
Addendum II: Couldn't resist adding this hilarious interview with Elon Musk regarding his time at Queen's (another good Canadian school). He decided against UWaterloo, but for entirely different reasons: