On a MacBook pro 5.2 with dual GPU GeForce 9600M GT /GeForce 9400M
LR 6.7 on Mavericks => Slideshow plays normally
LR 6.7 on Yosemite (clean install) => Play and Preview buttons don't do anything (but are not greyed out). Solution: Apple default GPU driver is the culprit. Install Nvidia webdriver according to Yosemite build number.
LR 6.7 on El Capitan (clean install) => Slideshow plays normally with Apple GPU drivers.
On this computer, LR 6.7 automatically disables GPU (performance pref pane). Adobe states that GPU acceleration is only used by the Develop module but there seems to be something going on in the Slideshow module too. So I thought I would share this info for those still using an older MacBook pro.