Scouting defense lineman is heavily impacted by defensive scheme. Teams that rum a 4-3 defense need lineman that are more fast twitch and can rush the quarterback from both inside and outside of the line, where teams that run a 3-4 defense need lineman that are stout against the run and any pass rush is a luxury.
The defensive line positions include defensive tackle (DT), defensive end in a 4-3 or 3-4 defense (DE) and nose tackle (NT). Only a 3-4 defense requires the position of nose tackle and a 3-4 defensive end is different than a 4-3 defensive end as they require more length and size. 4-3 defensive ends are primarily tasked with rushing the quarterbacks, in a 3-4 defensive that is the job of the rush linebacker.
When evaluating players along the defensive line you should look for some similiar and other different attributes at each position, so I am going to break it down by position along the line.
Nose tackles need to be big, strong and stout. This is one of the few positions where height can be a disadvantage, as shorter players will be able to utilize better leverage to hold their ground. Ability to be stout in the run game is key, the job of the nose tackle is to take on blockers so the linebackers can be free to roam in space. Lateral quickness is an asset as they need to be able to advance to the side of the runner and not have their face crossed by blockers. Strength, Size (300+lbs) and short space quickness are the key attributes of successful nose tackles.
Must be able to stop the run up the middle either by holding their ground or penetrating into the back field. Defensive tackles are expected to push the pocket back and collapse the space around the quarterback, as well as be able to stop the run. Size is important, as they will be required to regularly take on 300+ lineman and split double teams. The ideal size for a defensive tackle is 275+lbs and 6’3″ tall as they still need to be able to get under the pad-level of blockers. Strength is required, however quickness and use of hands to shed blocks is more critical. Lower-body explosiveness is also an asset as it should translate to the ability to push the pocket.
3-4 Defensive End:
Very similiar to defensive tackles, except for the ideal length that is required. The majority of 3-4 defensive ends of the league are closer to 6’5″. Another difference is that 3-4 defensive ends need to be able to occupy blockers and keep their leverage in the run game, they are not required to rush the passer (other than on the occasional stunt). This is a position where strength, size and technique are prominent.
4-3 Defensive End:
4-3 Defensive ends are required to get after the passer and be able to maintain the edge in the run game. This is a position with elite athleticism is paramount, the player must be able to turn the edge on the offensive tackle and have a variety of moves to get to the quarterback. They are rarely required to take on a blocker right off the snap, but have the freedom to go around the blocker and make plays in the backfield. 4-3 defensive ends must be able to tackle and ensure they keep contain in the run game, but they are not required to be elite run defenders. Explosiveness and speed is what you are looking for in an elite 4-3 defensive end.
The most dominant defensive lineman in the game today is J.J. Watt, DT, Houston Texans. He is a 4-3 defensive tackle, but has the ability to play every position on the line (except for maybe nose tackle, but why would you put him there anyway). He has elite athleticism and explosiveness, a variety of pass rush moves, and incredible functional strength. He is the gold standard of what teams look for in an defensive lineman.