Shorter stems are better going down and longer stems better going up. The shorter the stem the 'twitchier' and more responsive the feel will be.
If your stem is too short then climbing steep trails will be more difficult. The front wheel will lift off the ground easily and the bike will be 'jerky' not holding a line that easily.
The stem angle will effect the height of your handlebars. Racing type XC bikes generally have lower front end positions and freeride/dh etc a higher one.
You will have to experiment to see what you like in both cases of angle and length.
Good idea in the experimentation phase to either find a bike store that will let you try a few different stems before buying one or if this is not possible buy something cheap so that if it is not right for you it won't be such a waste of money.
Not much for a man to ask I dare say.. the simple maturity to ensure a limitless supply of clean socks.