When making your comments, please consider discussing the following things. You don't have to discuss everything from this list but be thorough, write in complete sentences and be thoughtful. The more closely you start to look at these things, the more trained your eye becomes.
Elements of Art - When writing, ask yourself, "How did the artist use this particular element of art"?
Line
Shape
Form
Texture
Space
Color
Value
Principles of Design - When writing, ask yourself, "How did the artist use this particular element of art"?
Center of Interest - Focus
Balance
Harmony or Unity
Contrast
Movement
Rhythm
Color
Complimentary
Analogous
Primary
Secondary
Cool Colors
Warm Colors
You also may want to ask yourself the following questions and answer them in your critique:
How does this painting make you feel?
What do you think is going on in this painting? Is there a story? If so, what is it?
Do you like the painting? Why or why not?
Winslow Homer - Snap the Whip, 1872.