If you see a traffic hazard that the city can fix - you can report it by calling (503)823-SAFE. The city has been real good about responding. They fixed the problem I reported in under three days! But there has to be a legitimate problem. You call, leave your name and information and a description of the problem on their answering system. If they need more info they will contact you. Once again, that number is (503)823-SAFE (503)823-7233.
If you have a parking concern. Such as cars parked across a sidewalk, bike lane, or a driveway - you can call parking enforcement. (503)823-5195. I even called them after hours and left a message and there was a response!
If you see Graffiti anywhere in Portland, you can report it to the Graffiti Abatement office. Usually it will be removed promptly... I have had good results with them as well.
Again, these services work and are a great thing for the city. But like anything else, if they are abused they become less worthwhile. Please only use them if you have legitimate concerns.