I venture that 95%+ of the readers here understand device drivers (at least from the point of buying something new for their computers), so they should understand the issue here without the need for analogies.
Each of these phones has different hardware, aka devices, that will require different device drivers to make the Android OS work properly with that hardware. Without said device drivers, the Android OS simply will not be able to boot on your phone.
If you load the N1 OS onto the DROID (without modifying it to work on the DROID) you're going to reboot your phone and ... nothing. hence the mention of paper weight mode. Your nice, hefty DROID becomes, in effect, a paperweight.
It is no different than trying to use ATI drivers for an nVidia graphics card - not gonna happen.