The question about what makes good subject matter for a painting always comes up in my classes. I my view painting breaks down into two basic philosophies that are polar opposites with many variations in between them.
One idea is the Subject with a capitol S this is usually something grand, dramatic or very complex. In these types of paintings everything is in service to the subject and handling is secondary. You see realists mostly in this camp. Their focus is on illusion and not so much stylistic interpretation in their mark making. Even though this is the case it is still a personal view and the individuality manifest itself in other ways.
The opposite idea is the handling is the goal and the subject is secondary. The handling takes an ordinary subject or no subject at all and makes it interesting. You see looser representational painters and nonrepresentational painters fall into this category. These painters like leaving visible marks that call attention to the process not the subject. The complexity is in the abstract arrangements of surface quality and color and edge.
Of course these are the extremes and there is everything in between too. Most people fall somewhere toward the middle of these ideas, where either subject or handling dominates but both are integral to the paintings success.
The importance of understanding this is to help the artist decide what kind of painter they are and guide them to what they love to do. To help them find where they fit between those philosophies if at all? Finding a personal view makes a better painter because a painting that is heartfelt and honest in its approach and interest will find some aspect of the truth of a thing. In my mind that is where all good painting comes from.