You can see on their Press Release they talk about creating 'true to life thunderstorms.'

With 6.5 it's now really very simple. As Wabe smartly said, you can now include lights inside clouds, and animate them on and off and they will create the illusion of lightning in the clouds. If you want a lightning bolt, just create one in Photoshop and apply it to an alpha plane with a jacked up luminosity and glow and animate it on and off at the same time you animate the light on and off.

That's what they meant by 'true to life thunderstorms.'

Before 6.5, a light object inside a cloud would not affect the cloud at all. Now it does, hence the ability to have a thunderstorm with lightning.