1. Hardware Anti Ailasing. Free Anti Ailasing. It's epic and such a timesaver it's beyond words.
2. Action Multi-Outs. Again, so awesome. Render out different cameras, different elements, different mattes, all in one action. No more expressions linking four actions for correct mattes. So great.
3. Nice lights. Lights are really beautiful in Action now, with soft falloff and much more control. My only complaint is you can't control whether or not they throw specular separate from diffuse light, but it's better than it could be thanks to:
4. Shaders. While it's cool that you can now add a variety of different shaders (really just different specular highlights or fresnel gimmicks) to your surfaces, the best thing is you can use a shader node to turn shading off per-surface. No more linking individual lights to try and brighten up a scene correctly. Good times.
5. Substance Noise in Action. I think this is deeper than it appears. The textures are nice, but not mindblowing and will certainly need some love before you can stick them into a photo-real scene, but there's a lot of useful shit embedded in them that's useful outside of making brick walls.