So, what exactly is poor form? What the other guy is doing?
I hear it a lot but honestly never see a stick figure diagram that tells people how to run.
It changes all the time. Last I heard (and I don't run so take this with a grain of salt) 'experts' have been saying that barefoot running can be really good because it encourages you to strike with the toes and ball of your feet instead of the heel.
Running is bad for your knees and joints.
Humans are made to run. Unfortunately, most of us have devolved away from our intrinsic physical capability.
Humans are made to walk. we are made to walk all day but still, mostly walk. We are capable of running and actually pretty good at it but mostly for short periods, not long periods. Horses are made to run. Long, flexible tendons between 'heel' and 'toe' allow alot of energy to be absorbed and reapplied in the next stage of the gate without putting alot of repetative impact up and down the spine. All the really good running creatures are quadrapeds (okay, kangaroos, but they bounce not run) so the spine is not compressed on every step. We can run, and I know lots of people that run alot and are healthier than they might be because of it. and good shoes can make a huge difference if your form is not picture perfect. but these are all methods for reducing the impact on your body, which if we were meant to run evolution would have taken care of already.