The main cause of soreness is usually damage to the connective tissue, which in turn causes an inflammatory reaction. The inflammation is part of an early triggering event for muscle growth. Eventually your muscles adapt, your connective tissue strengthens and the soreness is reduced.
All of this is necessary for your muscles to adapt because adaptation = growth.
But let's remember that connective tissue does NOT adapt as quickly as your muscles - steroids or not
As to why legs get it more than other muscle groups?
My educated guess is similar to bigben's view on this - your indirectly flexing your legs constantly in day-to-day life when walking, standing up, etc. This means you'll inevitably feel more soreness because all that connective tissue is being used all the time - despite the fact that it needs rest.
For example, do you notice that when laying in bed the soreness in your legs is completely gone or at least dramatically reduced? That's because the connective tissue isn't being used - its recovering.
If we compare to other muscle groups:
When are you actually flexing your chest during day-to-day life unless your pushing a lot of stuff? When are you flexing your back unless your pulling a lot? These muscles and the connective tissue surrounding them are hardly ever engaged for a sufficient amount of time during your daily life for you to feel any extra soreness.
To answer another question Schredder PM'd me, training a muscle group more often always reduces muscle soreness in the long run (that's why you felt it less man). The guys who train 1 muscle group per week are ALWAYS in more pain than those who do it twice or more - and let's remember that pain/soreness does NOT equal growth.
I know your a fan of constantly changing your workouts too man - be aware that this always leads to more soreness because your not giving your body, and the connective tissues, a chance to adapt & strengthen.
I know some people believe in the muscle confusion & not giving your body a chance to adapt, I've made my feelings clear on this nonsense before, but think about this - if your body doesn't have a chance to adapt, how is supposed to get stronger & grow?
Again, just something to consider for the future dude.
As a side note, I know some guys like using ice/Epsom salt baths, etc to reduce the soreness and that's absolutely fine PROVIDED your not doing it soon after your workout has finished.
The reason for this is simple: as I mentioned earlier, if you stop the inflammation your also stopping your body from adapting and reducing the maximum potential for muscle growth. All of this also means that you will feel sore after every workout simply because your connective tissues don't have the chance to adapt & strengthen.
Feeling sore after legs is fine, especially after a high intensity workout, but if you feel sore after every single workout - this isn't normal and should be addressed.
You will still make gains but if your connective tissues aren't able to ever adapt - your likelyhood of injuries is much higher, as many enhanced guys know all too well.
