10 Mistakes in Icon Design

1. Insufficient differentiation between icons
2. Too many elements in one icon
3. Unnecessary elements
4. Lack of unity of style within a set of icons
5. Unnecessary perspective and shadows in small icons
6. Overly original metaphors
7. National or social characteristics not being taken into account
8. Images of real interface elements in icons
9. Text inside icons
10. Outside the pixel framework

10 Mistakes in Icon Design by Denis Kortunov

Update: also check out Apple Human Interface Guidelines for some icon design insights.