Simply put: heavy = fast and lots of spin. You really need both factors to qualify as a heavy shot, though weight of shot is subjective and only really relavent to the person trying to return it. Flat, hard shots aren't typically considered heavy because you can just block it back. A slow ball with spin isn't heavy either because, since it doesn't have much speed, it doesn't skid through the court at all and uses up a lot of its rotational energy in the bounce, which will make it bounce high, but with little juice left once it gets to you. A heavy shot feels like it pushes your racket around and may even cause it to twist in your hand. I have hit heavy serves that nearly knock the racket out of my practice partner's hand. If you hit with pace and spin, then the pace will drive the ball through the court and kick it up higher (anything near the shoulders will inherently seem heavier just because of the height), but since it has more speed, it won't use up all of its rotational energy on the bounce and will still be spinning away once you hit it. The spin is what really makes a heavy shot bite the stringbed and makes it twist and cause the ball to just jump off the racket rather uncontrollably. It's all in the mechanics, but it definitely requires a good deal of racket head speed to produce a truly heavy shot.
Fear is a waste of time, and pain breeds fear -- ignore pain and live life.
Wilson ProStaff 85, Iso-Speed Control crossed w/anything cheap @ 60lbs.