I agree with those that bemoan the downhills, they are much worse on the knees than the uphills. I would never take a step on any camino without two poles whether I'm going up, down, or flat, because the repetitive stress on the joints is brutal, and the poles take some of the weight off the knees and other joints.
I'd venture to say that most of the camino walking injuries I've seen, though, have to do with something much more banal -- the asphalt. As the Camino is more and more paved over to tolerate the hundreds of thousands of footsteps that go by every year, there is more and more opportunity for tendonitis and other repetitive stress injuries. I have seen many pilgrims who are far fitter than I (many of them long distance through hikers) who got it bad. They are the folks who can walk tremendous distances easily, but forget that their experience is on unpaved, uneven dirt or rocky trails, where the stress is far far less on the skeletal and muscular structure. So, be careful of things like shin splints.
I know this is another one of my soap boxes, but I always carry a plastic bag so that I can get a bag of ice to soothe my tendons when I'm relaxing in a café. I also soak my feet in any stream or river I come across. And I obsessively hunt for the unpaved dirt path that is frequently worn into the ground on the side of the pavement. In some ways it is easier to walk on pavement, but in so many more ways it is a big cause of big injuries.