D5 has been nearly perfect for my workflow, but the area where I keep running into limitations is with materials, especially when a project needs more control than the standard maps we have right now.
In programs like V-Ray, realistic fabrics are done by using anisotropic reflections along with falloff or fresnel maps that control how light behaves at different angles. That combination is what gives velvet, satin, and silk their soft highlights and directional sheen. Having something similar in D5 would make it a lot easier to push those materials closer to real life.
Of course, these tools wouldn’t replace the simple approach that already works so well. They’d just be extra options for people who want to go further. A falloff map and a sheen control would make fabrics more convincing, and the ability to layer on things like dust, fingerprints, or smudges would bring glass, metal, and other glossy surfaces to life in a subtle way.