I think Chick is right about the stance, but I think the problem with the smith is the compressing force you put on your spine. You have to push against the bar, so unless you can align yourself perfectly, you are not going to be pushing 100% against the bar in the direction it is supposed to move, but in a slightly off position. If your feet are far in front, you are guaranteed to be pushing against the bar horizontaly as well as vertically. I did them with my feet in front of the bar for a few months and the stress on the knees was pretty bad. Doing them the other way put too much stress on the spine.
I don't do smith or free squats at all anymore, except occasionally front squats.. Unless you have the right build for them, they are just an injury waiting to happen.