[Cavernio]

I *think* the reason why such regeneration is hard is that there's no template in our genes that would allow these cells to grow into a human arm properly. Maybe you could grow a reptilian arm or something :-p

Speaking about our nerves specifically, one of the reasons why brain damage is so detrimental is because once neurons die, they have difficulty growing back to the same place. It's not that neurons can't grow, but that they won't necessarily grow the same axons and dendrites as they used to have, ie they won't make the same connections as they used to have. If we could manage to grow a human arm onto a human body, in order for it to be useful, you'd have to innervate it properly too, and as of a few years ago when I was in school, there was no way of reliably getting neurons to connect where you wanted them to be on an adult body.