Good Paint versus Bad Paint
If you are a professional painter. it is pretty easy to tell the difference between a good paint and a lower quality paint. Quality pants are just easier to work with than lower quality paints. They are easier to cut with, they cover better, they go on smoother, and they just look better when they dry.
However, It is not always easy to convince home owners that going with the higher priced paint is worth the cost. Some think that "paint is paint," and there really is no difference between a $25 and a $50 can of paint. From my point of view, and that of most professional painters, I often find it hard to understand why a home owner would spend their hard earned money to pay a professional painter and then use a sub-par paint. This is especially true when a home owner is only painting one or two rooms. The price difference between a good paint and a inferior paint need for an average sized room is almost negligible.
And, I do understand the perspective of the home owner. A wall painted with a inferior paint looks just as good as a wall painted with a quality paint, The big difference for the home owner is that the wall painted with the quality painter will look better than wall painted with the low-quality paint much longer. And, if there needs to be touch ups, they will look better (sometimes almost not even noticeable) if a quality paint is used.
To understand what makes a high quality paint check out the document created by the Paint Quality Institute http://www.paintquality.com/media-center/paint-publications/education/module4.pdf