I think the eccentric portion is what actually induces mechanical damage on the muscle fiber, or the majority of the damage in a given lift. This is when the fiber is pulling apart rather than contracting together (not exactly in every case). I know that the nervous system undergoes much more stress in an eccentric portion of the lift than the concentric.
yar, like when you basically "drop" the deadlift at the top position. Olympic weightlifting is really a great example (clean, push presses, jerk). Muscle-ups and dropping from the top position gives great results. You can do isolation work with ropes attached to a weighed sled, or legs/whole body by pushing it. Eccentric-less works best with a higher frequency and moderate volume because it isn't as "stressful".
I did eccentric only for a few months and maintained great development on it, mostly muscle ups, deadlifts, jumping and push presses. I think it would be hard to build a great physique with just concentric phase though. Probably sensible, regular weightlifting is best for hypertrophy
No not all, soreness may actually be indicative of less development taking place. Lyle mcdonald has some great articles on the topic
Bodyrecomposition - The Home of Lyle McDonald : Bodyrecomposition