What new features would you want? I still use Tiger on my primary machine and haven't missed much (outside of performance boosts) in the subsequent releases.
There was a time when Apple's ability to ship a new OS every 1.5 years was touted as one if proudest accomplishments. MS was insulted for years over their inability to ship a new version of Windows.
Desktop computers (and laptops) aren't advancing at the same rate that they used to. Also, OS X has matured tremendously since its reboot of the Mac OS. Maybe there's just not that much left on the queue.