Modern - characteristic of present and recent time; contemporary; not antiquated or obsolete.
A modern OS, is an operating system that is up to date with the latest usage trends and application requirement.
Today, that would be an easy to use and intuitive system with a touch-compatible interface and the ability to run 3d-accellerated applications at HD-resolutions.
That being said, being modern is not the same thing as being good, efficient or easy to tinker with. :-)
