Black Levels & Shadow Detail
Black level performance and handling of dark shadow details are closely intertwined. A projector can have so-so black levels but very good shadow detail, and you end up with a dark scene that is very flat looking. Or, you can have great black levels and just "good" shadow detail, and you end up with the opposite, a dark picture with a lot of pop, but some areas may show a little less, or no detail in a particular area, whereas that the other projector does. I think most prefer the second example. The slight loss of shadow detail and great blacks is typified by the Epson Home Cinema 6500UB. The Mitsubishi HC5500 would be a good example of the first scenario. Ideally you want exceptional performance at both.
That brings us to the RS20. Its black level abilities are unmatched. Richer and darker blacks than anything I've seen. The shadow detail, on the other hand, is simply typical of very good projectors out there. The combination of the two makes for the best looking dark scenes I think you can hope to find today.
First is a seriously overexposed shot of the starship in The Fifth Element. Note, that even with this ridiculous level of overexposure, the blacks in the image, and the letterbox are still not much brighter than pure black. Immediately below it, is a less overexposed version, for better comparing with the same image in older reviews. In this first image, I have left in part of the letterboxing, so you can see the basic black level more easily.