For some reason, for my reprap to work I had to plug my min opto end stop in to the max port on the stepper motor controller. So for other reprap’ers out there whose symptoms include: normal response to Gcodes, except for motion towards the min opto-endstop, for one or more axes, try inserting the min opto into the max opto port on the stepper controller. Makes no sense, but bam there it is.
I also had to invert my opto-endstops by setting this in _init.pde:
#define SENSORS_INVERTING 1
I spent days struggling with these idiosyncrasies, hopefully this post will save someone else the same trouble.
Also remember to set the baud-rate to 19,200