OK, going back to the thread title "Let us teach about fabric quality," what exactly DOES determine fabric quality?
There are three major quality factors in determining overall fabric quality;
1) The quality of the base goods (greige goods) upon which the fabric is printed or dyed.
The quality of goods is where it starts, and that quality is determined by a number of factors. The thread count is one -- most cotton made for quilting is what they call 60 square, i.e. there are 60 threads in each direction of the weave, per square inch. Some higher-quality quilting fabric will be 75 square, and batiks and hand-dyes tend to be on even higher thread count fabric, which is what makes them a little more difficult to hand-quilt.
However, thread count isn't the only thing that makes a difference -- the quality of the thread used to make the fabric, and the thickness of the individual threads are also important factors. Cheaper fabrics may have a 60 square thread count but be made of cheaper, rougher lower-quality thread that is also thinner. Less care is also given in the weaving process in cheaper fabrics. The higher quality fabrics will be printed on goods made from more rare, long-staple cotton.
2) The quality of the engraving and printing.
Quality is all over the board in this part as well. Hallmarks of high-quality fabric are the care taken in the design process, the engraving process and the printing process, and the quality of the materials used in the process. Cheaper fabrics will be quickly designed (or even copied) electronically with not a lot of care taken, the engravings may be sloppy, and cheaper dyes will be used in the printing. Fewer color overlays (screens) will often be used. The overall result is that the fabric will often be muddy or blurry looking with not a lot of crisp lines and will be less colorful. The dyes will often lack brilliance and the fabric will just look sort of boring. The designs themselves will mostly be less intricate and simpler.
Higher and highest quality fabrics should have true colors, crisp designs and a lot of obvious care taken with the printing process. The highest quality fabrics will sometimes come from original paintings (although this is getting very rare, only a few companies do this anymore), engraved with the greatest care, and with a lot of attention to detail in the printing process.
3) The quality of the finishing.
Quality finishing, along with the previous two points, determines how the fabric will feel. Does it have a nice soft hand? Is it easy to work with? Does it have enough, but not too much stretch? How does it feel when you touch it and work with it?
The highest quality fabrics will have a great "hand," will be easy to work with, and will be pliable yet hold their shape when sewn. Lower quality fabrics may feel rough, have a lot of obvious "starchy" feel, and may be stiff or will excessively unravel.
Hope this helps you teach yourself what to look for when you're fabric shopping. In most cases, you get what you pay for. There are of course bargains to be had everywhere. I encourage you comparison shop, but make sure always that you're comparing apples to apples, etc. Just because you pay a high price, doesn't always mean you're getting the highest quality fabric. But if you're paying an ultra-low price on fabric purported to be high-quality, let the buyer beware...