RDRAM killed itself by being proprietary, charging royalties, and suing the hell out of manufacturers.
PCIe as I understand it is an open standard by none other than Intel, and PCIe is freely avaliable to anyone who wants it.
PCIe isn't shared bus as other people mentioned. So every card gets it's own dedicated 150MB/s up and down per lane.
Also, bandwidth can also be scaled to the application. You could probably run something on a PCIe 4x slot and not incur a massive performance hit, or if you're hitting the PCIe bus with everything you've got, build a 32x PCIe card with the slot to match it. It's much more scalable than any technology we've seen thus far.
It's also interesting that PCIe cards scale up just fine. So you could put a PCIe 4x card in a 16x slot, thus ensuring future compatibility. Or you could put a PCIe 16x card in an 8x slot built to 16x slot physical specs, ala SLI.
PCIe also has the potential to be external tech, too. Can you say external graphics module? Or HDTV Module, more realistically.
It could also probably scale in frequency meaning it could just scale up into infinity.
PCIe is really exciting for me, if you can tell.