There are four offensive skills and three defensive skills. The offensive skills are hitting for average, hitting for power, strike zone judgment, and offensive speed. I consider strike zone judgment the most important of those, in the sense that most good hitters control the strike zone, while most bad or mediocre hitters do not. Generally speaking, the more walks the better, and the fewer strikeouts the better. Ideally, you want someone who draws a ton of walks and seldom strikes out.
Few players reach that goal, of course.
I don't mind someone who strikes out a lot, as long as he draws walks as well. Other guys are adept at stealing bases through guile and skill, rather than pure speed. The three defensive skills are range, reliability, and throwing utility. I consider range the most important of the three. A young player with good range but weak reliability has a chance to get more reliable as he gains experience.