Hi,
good question, I've wondered about that myself sometimes. I consider myself a pretty seasoned exerciser too, and I don't get sore very easily. I too was wondering if I'm working hard enough, since some people seem to get sore much easier. It seems to me like getting sore varies from person to person just like everything else related to exercise. And most importantly, I do seem to get great results (THANK YOU, Cathe!).
I have noticed that instead of being excessively sore, I often feel a certain "tiredness" in my body after tough workouts. Also, surprising things can suddenly get my muscles sore, though like you described, not anything like "not being able to walk". Like the previous poster said, concentration does play a big role there, some days you have it, some days you just can't quite get there...
Hope this helped!
Maria