You're asking for some things that may not be technically feasible. In particular, to have a larger sensor, you need to either have a larger lens, or you need to have a less that doesn't open as wide. Part of why the fixed-lens digicams are so nice, is that, by using a physically small sensor, they can engineer custom optics that provide lots of light without taking up much space.

Another issue is the absolute size of the pixels. When you get physically small enough, diffraction issues start becoming a real problem. It's difficult to focus visible light smaller than a certain size. So, to have higher resolution, you're forced to have a physically large sensor, and that requires larger lenses. But, you still have to worry about fabrication defects, so the yield will always be lower (and the cost higher) on larger sensors.

Likewise, some of the other features you're asking for (particularly larger memory buffers) are expensive and won't be a feature that a non-technical customer will ever care about. Only the sports photographers really want those features, and they're buying top-of-the-line D-SLRs. Those giant lenses weigh so much that the camera's weight is irrelevant.

I actually think Nikon's DX standard is a good idea. By having a smaller image circle, they can engineer smaller and lighter lenses, selling you amazing toys like a 10.5mm fisheye lens for $600 or so. Nobody else comes close to that. It's a whole lot easier for Nikon to do it when the lens doesn't need to produce a larger image circle.

I agree that it would be nice to have smaller and lighter SLR cameras. Of course, you're welcome to get fixed-lens cameras, many of which are quite good, but they're not aimed at the high-end professional's needs. Olympus D-SLRs are noticably smaller than their film cameras, having the lens all the way on one side. So, at least one vendor has heard your complaint. Still, I imagine that Nikon and Canon are happy to have all that internal room to cram in all the extra electronics that they don't need in a film camera. I doubt there's much open air.