This NT has to be a real monster of a guy, 350 pounds or so, because his job is to take on the center and one of the guards simultaneously on every single play. Then you get two more defensive tackles at around 300 pounds each, and play them up against the offensive tackles. All three of the defensive tackles have what is called two-gap responsibility. They are expected to hit the offensive linemen head on, and watch the play to make sure the running back doesn't come through on either side of them. Also, they're expected to hold their block so that the offensive linemen can't get out and block a linebacker.
In the 3-4 system, the linebackers are expected to make most of the plays. In the 3-4 system, the DTs play a more physical game as they are taking on one or two offensive linemen directly, play after play. Unlike the DTs in the 4-3, the DTs in the 3-4 are responsible for every single gap in the offensive line. Although the DTs get relatively few chances to make tackles or sack the quarterback, anything bad that happens is still ultimately their fault.
In the 3-4, you have four linebackers. Two of these guys are inside linebackers, and are expected to weigh roughly 240 pounds and be quite athletic. You also have two outside linebackers. These guys are sometimes called "tweeners," as they are in between the normal size of defensive ends and linebackers. These guys should weigh perhaps 255-265 pounds and also be quite athletic. Because these are linebackers, they tend to be faster than the heavier defensive ends. Their presence makes it much more difficult for the quarterback to roll out, as he will be rolling out directly into the path of one of these linebackers.