I like the LISA concept. It can be REPRAP or REPSTRAP, but it is really the small number of moving parts and linear print area in the Z like other popular deltas that give it extra appeal. The one thing that seems to be a drawback is the high cost of the quality threaded rods and nuts. There is also that thing about sliding motions.
The threadless screw idea previously put forth would seem to be an answer to the cost hit, and no sliding motions. I am sure there are some downsides to the practical implementation, and many questions, like:
1) How large a thread "pitch" is practical on an 8mm rod?
2) Can 3 ball bearings be the whole bearing solution for each rod?
3) How much variation would there be in "thread pitch" with practical tolerances?
4) Could each of 3 rods be custom calibrated to its natural "thread pitch"?
5) How much cost would it really save?
~Dennis