Police
Usually you don't have anything to fear from the police when hitchhiking.
Though there are some areas in the world where it's forbidden to hitchhike, such as a couple of states in the USA and according to some cops in Australia it's forbidden in that country as well (but possibly only in their jurisdiction, and only because they say so).
In China the police generally acts really confused but extremely friendly. Sometimes they even offer free bus rides (which are usually shittier than hitchhiking cars though).
In countries with an elaborated system of highways you should avoid walking on the highway. If you do, be prepared with some kind of excuse.
In France it's mostly a bad idea to be on the highway, but standing close to the Peages is mostly tolerated - unlike in many other countries such as Italy and Spain.
