Jeremy Lin's "D" is light years ahead of Nash. (The reason why I think Nash has never been able to win a title.) To give you an idea, Lin had three blocked shots in the game in Chicago which Derrick Rose aggravated his groin injury. (The injury occurred on one of Lin's blocks.) Nash never had three blocked shots in a game in his entire career!!
Lin has also a five steal game, which Nash achieved only once in his career. Nash's career has spanned 16 years.
Lin also sets screens and boxes out for rebounds. A lot more active away from the ball.
I personally think Steve Nash was overrated because he never really did much outside the statistical realm. He never was a good defender and always played passive without the ball. (I don't recall Nash ever diving for a loose ball or making a hustle play. Not exactly Rajon Rondo...... Could be why Nash has been able to last a long time.) Jeremy Lin on the other hand will run through back screens, and be the finisher as well as the facilitator.
Whoever gets Steve Nash next season will marvel at his assist numbers, but scratch their heads over why the team doesn't win.