There is the brute strength that allows him to overpower opponents, the athleticism that leads to his big plays and an unmatched intensity and relentlessness that has made Smith one of the most unheralded stars in the game.
Coach Jim Harbaugh called Smith the "toughest player" he has ever been around in his long career as a player and a coach.
"You talk about 10 out of 10. You can't get any better, can't be any better than 10 out of 10," Harbaugh said. "Then, he finds a way to take it up another notch on the ladder when you didn't think there was another notch to go. ... The play where he's hurrying Drew Brees while he's being locked out by their left tackle. Legs off the ground and he's holding on to Drew and pulling him down. Is there an 11 out of 10? Is there such a thing? Is there a 12 out of 10? He just keeps finding another rung."
Smith has been doing that all year.
There was the play where he chased down the speedy Jeremy Maclin from behind to force the fumble in the closing minutes to seal a 24-23 victory at Philadelphia in Week 4.
Then there are the litany of other plays he made on a weekly basis to help lead San Francisco to a 13-3 regular-season record.
"I knew he was a good player," defensive coordinator Vic Fangio said. "He's even better than my impression was. And I think what a lot of people forget about Justin is they fall in love with his tenacity, his effort, his toughness, which is all justified, but he's got talent. He's a talented player. So, when you combine the intangibles that we spoke about with his talent, we get what we see this year."
Smith played in just one playoff game in his first 10 seasons, losing to Pittsburgh with Cincinnati in the 2005 season. That led Fangio to put him on the cover of the defensive playbook, with the tag: "0-1."
Fangio wanted to motivate the unit to get Smith his first career postseason win and he got it last week with a 36-32 victory over New Orleans. Now Smith is one win from a long-awaited Super Bowl trip.
"The opportunities are far and few between sometimes, and you got to make the most of it," he said. "We're here right now and, as a team, we just have that mindset of, let's make the most of it, go out and try to take it."
Overlooked for much of his career, Smith got some just due this season. He was picked earlier this month as a first-team All-Pro at defensive tackle and a second-teamer at defensive end - the first player ever picked at two defensive positions in one season.
With most attention on defensive linemen who pile up quarterback sacks, Smith makes his contributions by doing a little bit of everything, whether it's being stout against the run, collapsing the pocket from the inside to create sacks for teammates or making his own big plays.
Smith had 7.5 sacks this season but did much more. He added 18.5 quarterback knockdowns, seven hurries and drew so many double teams that rookie Aldon Smith got 14 sacks of his own.
"He's just a bulldog," former All Pro defensive end and Fox analyst Michael Strahan said. "I love watching him play because he's just tough. He's not a guy who will run around you and dance around you and look pretty. He doesn't care about that. He's just going to beat you up. That's always the best kind of guy to play with and the worst kind of guy to play against."
Strahan said he and Hall of Famer Howie Long spent a lot of Sundays this season in the Fox studio marveling at Smith's play and talking about how he might be the defensive player of the year.
Strahan is happy to see more praise go to an all-around lineman as opposed to a speed rusher worried only about getting to the quarterback.
"It kind of gets old to talk about a guy who doesn't really want touch the ground or run into anybody or be physical but just get sacks by running around all the time," Strahan said. "Those are the guys who get attention as far as the defensive linemen are concerned. It's good to see him get that attention. Maybe all the focus will go to someone who's an all-around player and not somebody who is one-dimensional. He deserves it."
Smith's teammates in San Francisco need no convincing. Smith has been the leader of what has been perhaps the league's top defense this season. He organized the defensive portion of offseason workouts during the lockout, sets the tone each day with his work ethic.
"He gets those guys going up front each and every day in practice, during preparation," safety Donte Whitner said. "It's his 11th year in the National Football League, and he looks like he's getting stronger and stronger."
Smith has started 171 consecutive games, the third longest active streak among current players and almost never leaves the field. He played all 80 defensive snaps last week and even got on the field early as an extra blocker.
He finished that game with three tackles, a sack, five quarterback hits and showed no signs of ever getting tired. His impressive bull-rush came in the fourth quarter.
"That's how I always played," he said. "I always played that much and I've always been on the field. I don't know how to do it any other way."