Most men will find that having the bar a little above the knee's yields the best results, both safety and power wise. If into BB'ing, than no need to start regular DL's from the floor. Another position would be mid thigh combined with the above the knee position. May also consider Romanian DL's, off a set of pins, as a good lat/back developer. There is not any leg push, but plenty of lower back/glute/ham action.
With Dl's, the upper body is holding the bar in a semi-stable position while legs do the actual pushing off. With the aid of the hips/glutes/hams of course. You are not pulling with the arms or anything else in the upper body. Which can show the effects of just holding a heavy weight as a muscle/power developer.
DL's, in any form, are a terrific back/trap/lat movement. But not meant for everyone. Gives a dense and thick total back mass that most exercises can not offer. Trouble is, a lot of people have a problem with the knee's getting in way of the moving bar (that and general sloppy style and using too much weight when not ready for it). Force to lean too far forward, trying to avoid touching the knee's, only opens the back up for more stress and over extending the lower area....equals injury's. Starting the lift from a higher position, above the knee's, may solve this problem for a lot of folks.
I do think the Smith Machine does have a place in BB'ing, but with lighter stuff. Regular DL's (and squats & BP's) would not fit well with it. When doing full ROM BB DL's there is a natural "S" curve to the lift (as with most compound exercises), like gears shifting position. The Smith Machine does not allow that, only a fixed or straight line of motion. Negative force on the joints, tendons and ligaments doing a heavier exercise. Though a Smith Machine may show some merit when partial or short range movements are done. Good Luck.
