There are a lot of things that go into the quality of a fabric.
For starters, there is the thread count. Many quilters prefer the "beefier" 60x60 fabric; most Modas (not their Marbles, though), Kona cotton, Timeless Treasures, and Michael Miller Fabrics use this and some other makers do also.
Other companies print on a finer 68x68 cloth. Moda tried this with a couple of lines last year but their decision was not well-received by quilters so they are going back to the 60x60. This finer cloth was noticeably thinner and the bolts were lighter.
Then there is the quality of the printing. How delicately detailed is the design? Does it look crisp, or sort of blurry? Do the colors line up? How many different colors are used to print the design? The more colors, and the more highly detailed, the greater the cost will be. Some designs require multiple trial printings - called strikeoffs - before it turns out right.
Also there are different types of dyes used to print the fabric. I've been told the best are fiber-reactive dyes, then pigment dyes, and last is wet-printing.
Finally there's the finish on the fabric. Some fabrics are silky smooth to the touch and some are rather rough.
When you're shopping for a fabric, you're going to go for a nice look and feel. Not too thin (especially for light-colored fabrics). You will probably come to prefer certain brands. The better brands usually are quite consistent in quality though occasionally they can have an "oops!" in the manufacturing process.