The biggest problem in this project has always been to create a processor that others can see what is going on, build it themselves and adapt it themselves.
This is quite a design challenge for me. I want all the components and wiring to be visible and I'd like the costs to come in at about £100 so that I don't put off others building and adapting. I believe this rules out standard PCBs.
I'm developing the idea or printing onto paper, sticking the paper on matrix board and mounting all components though the paper. All wiring is then visible on the front surface. I am allowing myself power supply wiring on the reverse, using map pins (or similar) on the visible side.
So far I like the idea. The white background gives good contrast and the finished circuit does look like the schematic, so making electronics visible, my main goal. Here is my first "PCB":
What I haven't got right yet is (a) what to print (b) I need whiter heavier paper (maybe plasticised) and (c) it's fiddly getting 4 legs down one hole. Oh, and it has to work of course, what you see above isn't soldered on the reverse, this is very much work in progress.
More detail at http://www.nanocpu.org/leds-and-wiring