Also, yarn fiber has a LOT to do with the way the finished project looks. Certain yarns and certain patterns just don't mix. That's why you have to consider what the look is that you are trying to achieve. For most scarves, fiber doesn't really matter. But for tops like form fitting sweaters, you're going to want yarn that has flexibility. For some of my favorite hats require yarn with some drape. Most slippers need firm yarn that can keep shape.
Just some ideas. Think about texture (soft, rough), think about flexibility (usually silkier yarns will have more drape, loser stitches are capable of more stretch).
Blah, hope I helped.