I think that "modern" in design means that you focus more on the heavy use of line to create geometric forms as opposed to using soft organic line creating more decorative elements found in more "traditional" styles.

Modern is more geared to function creating form and clean sterile spaces that leave little, if at all, notion of overstimulation. Wide open floor plans and clean lines are what contribute to the idea of modern design for interiors. As for the exterior, you still see clean lines and geometric forms as well as monochromatic and neutral color schemes.

In this space, geometric shapes, straight clean lines, and lack of a multiple color scheme gives this room a more "modern" vibe.

http://www.yourmomhatesthis.com/modern-furniture-how-to-achieve-that-contemporary-look/