Experience and time spent on a craft is no doubt the key to this question. The rule is that it will take about 10 years to master a skill.
Whilst experience is certainly crucial, there are a few aspects that differentiate the layperson from the professional. Next time you go to a museum, see if you recognise any of these traits. You can see where artists deviate from these principles and where they adhere to them. And if you’re on the other side and painting yourself you might want to try any of these – they are art school approved!
Proportions are important and reflect the time that a) the artist has spent looking at the subject and b) the time they spent constructing the image and composition. This is especially important when painting portraits or human bodies, because we humans are highly attuned to seeing and recognizing faces and movement. If the proportions are off by even a little the subject will look weird.
Many artists don’t adhere to these proportions later on in their career. A famous example is Pablo Picasso. He was so intimately familiar with proportions that he noticed the things that were different and accentuated them. He distorted real life proportions by emphasising where they deviated.
By depth I don’t mean the perspective of a room but a depth in colour and complexity. This element is super important for abstract art and can make or break a painting.
Depth can be implemented through super-imposed layers, much like the paintings of Mark Rothko. He thinned his paint and applied hundreds of layers, one over another, in order to give his colour fields the impression of floating.
Another technique is to paint the canvas a colour that is not white after having primed it. White is not a great colour to start a painting with, which leads me to point number three:
3. Black and White
Most professional artists use black and white paint in a very limited fashion. This is because it changes the value and temperature of the colour. By temperature I also mean the feel of a colour. When I add white to red I get pink, a very different colour from the one I started out with. When I add black to yellow it turns into a greyish-brown sludge.
This happens to all colours! It might not be quite so noticeably visible but the look of the colour changes and it creates a sense of flatness in the painting. For bright colours use bright pigments. To darken a surface use a darker shade of the same colour or a complementary one.
However, this doesn’t mean that black and white need to go into the bin. They just need to be used in moderation without relying on them for light and dark!