Pretty much every lens since World War II that can be focused, can be mounted on the X-cameras. Novoflex have some mounts for very rare lenses, and there are a wide variety of low cost adapters from China on eBay. The Metabones Nikon G Lens to Fujifilm X-Mount Camera Speed Booster is now available, so if I get one, all my Nikon lenses will match their field of view on the D700, but also pick up 1.0EV in speed.
I have a mount that should be here any day now, to handle my old Canon RF lenses, and I plan on getting a Nikon adapter as well for my X-Pro1. There are a couple of high-end Zeiss lenses now in the XF mount and Samyang/Rokinon/Bower have an ever growing catalogue of manual focus, but highly regarded lenses in the native mount. Meanwhile the catalogue of excellent Fujinons continues to grow.
I greatly appreciate the level of support. Even though the X100 dates back to 2010, there is a strong rumor that a firmware update may come through as soon as the end of the month. With the X100s on the market, most other camera companies would have long forgotten the previous model. I was very happy with it on day one, but must admit that it has been getting better and better.