"Modern" is not about techniques, it's about aesthetics. Some hallmarks of modernism are: minimalism, simplicity over complexity, pure geometric form over traditional decorative motifs, exposing the materials used (for example large swaths of solid color where you just see the cloth and the quilting pattern).
Any one of these things might have existed in previous eras, or be part of other aesthetic movements, but taken together they make up "modernism".
Most people making modern quilts don't think about these things consciously, but it is within the consciousness of all of us as part of the culture.
Some of what I see being called modern quilts I think would be more accurately described as postmodern. We are in the postmodern era currently so most of us have some postmodern consciousness without realizing it. Postmodernism is defined by a willingness to use any technique, from any era or culture, and mish-mash them into something new (and often discordant-looking). It's about creating new forms out of deconstructing old ones.
So a typical thing I see people doing is taking traditional elements and incorporating them in their quilts, but with a modern color scheme or overall modern feel So that is actually postmodern, technically.
But all of this is aesthetic theory - if you like to quilt, I say just quilt what makes you happy (which is, of course, a very postmodern thing to say).