Guest,John seems to have nailed most of it. What he did not talk about though is all of the little tiny things that can "sometimes" (read EVERY time) go wrong. Little things like flat batteries in someones guitar, yeah the same someone who just changed it yesterday. Every connection you can name is a potential non-connection waiting to happen. There are hundreds if not thousands in an average setup, 12 in each connected xlr cord for a start and thanks to Mr Murphy we know when disaster will strike.
It is also one of those fields where people think that they can do better than the "idiot" running the board. Many times that is not the case, because they don't even know what is going wrong. Could most wannabees have actually patched the whole system together to start with. Do they know the difference between an insert and a send, where do the cords for the EQ go, is that an input or an output.
Have some sympathy for a decent sound person and "PAY" them for it, you will probably get what you pay for. Very few ameteurs would hack a day in the the real world, hell I know I would't.
JohnB