What's your response? Ask that question 100 times and you'll likely get answers that are about 90% ambiguous. It's not surprising, since what pleases the viewer is very difficult to describe in verbal language, as well as highly subjective and dependent on the development of the viewer's eye. As a contemporary plein air impressionist/colorist, though, here's a list of what I respond to in a plein air oil painting. I'll keep my list under 10 to avoid writing a dissertation and include my major concerns in looking at a painting.
1. Mood. (How ambiguous and subjective is that?!) I want to feel what the painter felt when she or he fell in love with the scene and I've noticed that I do when numbers two through eight are in place. (Number nine is just a lovely surprise and sometimes the nudge that starts me raving to others about a painting.)
2. Edges: Hard and soft edges according to the light and subject.
3. Hues and Values: Strong and well-depicted hues underlying the masses with close attention to the related values of those masses, so that the painting conveys the structure of color-in-light.
4. Mystery: Some of the subject almost seen: I don't want to see a photographic depiction or over-painting in any part of it, a totally blurred impression when I stand back from it or parts of a painting that aren't in synch with the rest of it.
5. Color Sense: Local colors approximately as greyed as in nature -- not much more, not much less.
6. Travel: Movement through the painting, including a focal point.
7. Perspective: It has to be believable. It doesn't have to be exact to work, but it needs to work.
8. Brush Work: Confident, fresh brush and/or knife strokes, but not slapped on without attention to all of the above or over-worked so that it becomes muddy. That's all I want to see in brush work, never manipulation of brush strokes simply for effect. (A couple of people have told me that this depends on how long the painter has been painting. I tend to agree, with the caveat that the painter must have at least a fairly firm grasp of the preceding list.)
9. Organicity: Even beyond not creating, for example, branches growing directly downward from the trunk of a tree, I love to see the surrounding area define the "thing" and/or the movement of the brush representing how nature is growing the "thing" and has created the scene as a whole.
If you love plein air, what's your list?