The biggest problems in their paintings come from a lack of drawing skills and little or no understanding of linear perspective. A simple understanding of perspective includes vanishing points, eye level and horizon lines and a station point.
Almost all of my effort in teaching goes to restating fundamental principles to my students, even those that have achieved some small amount of success. Many of my students have always been interested in art but did not pursue it as a career and so many of them lack the basic fundamental skills needed to create the proper framework to place their paintings over. Students often worry about developing a style but in my opinion style is irrelevant when the fundamentals are lacking.
Anything you paint that has volume to it needs linear perspective to accomplish competently; Portraits, still life, landscapes, seascapes, city scenes; all of them need a thorough understanding of basic perspective. The more complex a scene becomes the more understanding you need. Think of all of the situations that come up in paintings that have groups of animals or people or reflections, shadows or anything with a complex structure to it. All those situations will need an even deeper consideration of perspective. Why go through your life avoiding those things or painting them badly because you lack the understanding to paint them properly?
People interested in learning more about perspective as it applies to your painting and drawing can find the information in the books ‘Perspective for Artists’ by Rex Vicat Cole from Dover books, ‘Drawing Scenery; Landscapes and Seascapes’ by Jack Hamm and ‘Successful Drawing by Andrew Loomis’. Of those three books the first one by Cole is only one that focuses on perspective exclusively. The other two cover it in conjunction with good drawing principles.