Melanie,
I've gotten mixed reviews about stems.
On one hand I've got someone saying that having it angled up is good, and someone else saying that one which sticks flat and straight out is better, easier to feel what the front wheel is doing.
What's your oponion on what makes for a good mountain biking stem?

The type of stem you choose for your bike may be affected by the geometry of your mountainbike. If you bike has a very short top tube, you might find you need a longer stem to be comfortable on the bike, or to achieve a better climbing position. However, if you are too stretched out on the bike, or too far over your front wheel, you will find it very difficult to be aggressive descending because you are not balanced behind your seat on steep descents.

I personally prefer mountainbikes with longer top tubes which allow me to ride a short stem with some rise. This way I can have all the stretch over the bike that allows you to climb well but also allows me to descend aggressively because my center of gravity is far back from the hub of the front wheel.

The rise of your stem and/or the stack height of your stem will depend on your own anatomy and the geometry of your bike to determine what is comfortable. Comparing the look of a dual slalom rider to a cross country rider will be a good comparison of someone riding for optimal control at speed descending and optimal speed pedaling. Depending on what your skills and/or weaknesses are, you want to set your bike up to take most advantage of your skills and soften your weaknesses.

There is no right answer to this question. I know that a lot of cross country riders are choosing short stems with moderate rise and riser bars, a much more aggressive setup to years ago when everyone rode flat bars and bar ends. Depends on what you like more.