I was trying to figure out what kind of clipless pedals to use on my mountain bike when the thread becane designated as a gun thread without having a gun discussion. I would like to know what to use.

My qusstions:
1) What should I expect out of clipless on my mountain bike?
2) What advantages do clipless pedals have over platforms? Disadvantages?
3) What advantages do platforms have over clipless? Disadvantages?
4) Which shoes and cleats do I want?