Area Render, hands down.

"I've made one small change to the iris material ... let's do a quick check to see how it looks before spending twenty minutes to render the entire scene again. In fact, forget rendering the entire scene again ... I'll just cut-n-paste the newly-rendered eye onto the old render in post."

However, the ability to save material collections directly from within Poser (and they can even be applied to unparented props) ranks up there, along with render wipe and OpenGL acceleration.

I've had Python support since Pro Pack, so I don't really consider that a P6 feature, myself. This has definitely been a banner year for Python, though, with the release of helpful plugins like ShaderSpider, Wardrobe Wizard, PuppetMaster, and Physics.