Typically when I think of 'modern' fabric the first thing that pops into my head is not your gramma's typical fabric! No more drab muted colors (mauve and seafoam anyone?!), dainty flowery prints, intricate detailing, or limited choices. Modern, to me, entails bold colors, contrast, daring color combinations, clean lines, simple detailing, and trendy. Birds and owls are hot trends right now, therefore I would consider that more modern. Polka dots are timeless but I would consider an organized polka dot pattern modern, or a polka dot pattern with more than 2 colors. Big polka dots are also hot right now.
Color combinations is another thing that I think dictates modern. A fabric with lime green, red, pink, and grey is not exactly something that sounds pretty on paper but when you see it, it looks stunning. Red and aqua is another hot color combination. Anything turquoise is hot too. Colors are bright and vibrant.
Certainly when you hear a designer's name it automatically sets off that lightbulb that it's a modern fabric. Other designers that might not set off that 'modern lightbulb' can most definitely be considered modern, I think there are some designers that can create both modern and traditional fabrics. And one thing to remember is that what's considered modern today might not be considered modern 5 years from now.
I also think that when you pair modern fabrics with some very traditional style patterns you can create a gorgeous masterpiece and we're finding more and more of that happening. My own Star Sampler Quilt Along is made using some very traditional patterns but as I'm putting together the tutorials along with the fabrics (I'm using Tula Pink's Plume) I'm stunned at how fabric choices can really up the ante so to speak.
When trying to decide if something is a modern fabric or not ask yourself a couple questions. How would I classify this color? is it bright, muted, drab? What about the print on the fabric itself? is it fresh? timeless? crisp? unique? Is it trendy?
And remember this, not all modern fabric has to break the bank. I have found some great quality fabric that fits all the modern 'standards' at Hobby Lobby for less than $5 a yard. You can also have fabric that looks modern and is of a crappy quality. Just remember, when swapping, use high quality fabrics!
I hope this has helped to de-mystify modern fabrics for you!