Puppy Linux's CD-writing capability
 

Was there something special and/or difficult in the way the Puppy Linux designers made the liveCD writable? Or is it a trick that the other liveCD distributions could easily be set up for, but just haven't been yet?

Nothing really special AFAIK. From what little I have discovered Puppy is created as a multisession CDRW or DVDRW. Multisession CDs allow one to continue writing data. When Puppy shuts down data is saved back to disk. I suppose other liveCDs could be configured the same way depending on existing unused disk space.

I would guess that there some limitations since the data is read back in to RAM.