Basically, X-Plane is not an early adopter of graphics technology. Because of the nature of the rendering we do, we can directly benefit from "more of the same", e.g. if you simply gave me twice as many objects per second or twice as many polygons, we could make the sim look a lot nicer. So we don't need to adopt new graphics technologies until they're proven in games that need them more, like first person shooters. We're a small company with no influence on the industry, so we write the tightest message we can and use new features when the dust settles.
(From a utilization standpoint, we also provide the best graphics to the most people by using card features that are going to become wide spread, so it doesn't make sense for us to gamble on vendor-specific extensions that might not become available to everyone.)
With that in mind, there is some cool stuff that people are talking about that maybe someday we'll get to play with:
- Irregular Shadow Mapping - given a super-programmable card, you can create a rendering scheme that optimizes shadow map creation to remove artifacts.
- Out-of-order blending - the graphics card resorts incoming geometry so that all translucent geometry is drawn back to front. Doing this on the CPU is expensive (and in X-Plane's case, we often just don't get it right at all).
- Multiple dispatch to multiple targets. Even on a big multi-chip GPU (a lot of modern cards are two cards stuck together) the only render to one screen or texture at a time, even if there are a lot of parallel elements. This is good for a few big complex scenes but not good for lots of small scenes. I'd like to see all vendors support dispatch to multiple targets - this will make things like dynamic reflection via environment cube maps potentially a lot faster.
- Voxel Octrees. This is the one I hear a lot about - basically it's a change from 2-d to 3-d data structures on the graphics card to manage fast access to large chunks of graphics data. (Shadow maps, z-buffers, and environment maps are all more or less 2-d data structures.)