"You know, I believe that cities are all about difficulty. They're about waiting: for the bus, for the light to change, for your order of Chinese take-out to be ready. They're about frustration: about parking tickets, dogshit, potholes and noisy neighbors. They're about the unavoidable physical and psychic proximity of other human beings competing for the same limited pool of resources….the fear of crime, and its actuality. These challenges have conditioned the experience of place for as long as we've gathered together in settlements large and dense enough to be called cities.
I would add one more thing to the list of difficult things that enhance cities: non-right-angle intersections. These haphazard junctions of vehicular, bicycle, and most importantly pedestrian flows create such interesting places that they themselves become destinations. Destinations to eat, people-watch, meet people, catch a bus or train, etc. One of my favorite intersections I've experienced is Place Cambronne in Paris, pictured below.
Images from here and here.