Short Answer: quality in subject, materials and process.
Quality in Subject
Quality of subject is the most subjective, no pun intended. Who is to say whether one subject is better than another?
I'm certainly not qualified to say. But luckily it's subjective. I find that if you are aiming for truth no matter how simply, someone else will see it as true and valuable as well.
My work is also centered around beauty and realism and so it is very easy for me and anyone else to tell whether my work succeeded in high quality of subject.
Quality in Materials
Quality materials are the easiest to fall in love with and the most expensive to commit to.
I've completely fallen in love with the tools of the trade: the way paint comes out of the tube and rests on the painting surface, the texture and feeling of expensive watercolor paper.
But materials also make a difference in quality. Nice paper is going to hold up under the paint and water applied to it without tearing or disintegration. Good wood panels won't warp and bend over as much over time. Over time fine art paints will retain their color better.
I also take materials into consideration when I make fine art prints. Each print is made on a heavy, matte finished paper which gives an expensive feeling. They are also giclée prints which means they are printed with pigment based inks. Pigment based inks have a wider range of color and vibrancy and they have little to no fading over time.
Quality in Process
Process, process, process.
Process starts coming into play before I even start working.
When I'm considering a watercolor painting I typically mount that paper on a wood panel. There's a particular glue I use to mount. When I'm considering a oil painting, the wood panel needs to be primed. If the wood isn't primed than it will rot over time (100 years?) .
After I do a drawing, I spray it with a fixative to avoid the lead smudging into the paint.
When I'm painting, I am adding layer upon layer to create the smooth, blended, richly vibrant painting you see at the end. Consider the oil painting image above. Before I painted the true colors, I have laid down base layers. I chose colors that would support my next layer. I am going to add blues, purples and reds onto her skin so I layered her with orange and white so that her undertones are warm.
Before I painted the red and yellow on the Cup Noodle, I painted in under layer of black and white. The cup is artificial, so the underlayer doesn't need to be warm. Doing a black and white base layer helps me to find the correct tones. If I'm trying to color match, I don't need to get confused over whether something is the right tone (light or darkness).
With each layer I am correcting past mistakes and building the best possible image.