You got it. Lack of hustle. I'd bench him for a week.
The real reason for the play wasn't LaRoche but the batter, Nick Johnson. When LaRoche last saw him before turning towards first, Johnson had slowed up as if accepting being out at first, but immediately once LaRoche turned around, Johnson sped up.
LaRoche would have made the tag if Johnson had kept pace, and to be honest, he had no reason to think he wouldn't, because there is a serious deficit of hustle throughout MLB.
The other day, the White Sox turned a (IIRC) 3-4-6 Triple Play. Runners on 1st and 2nd got caught when Konerko charged in and caught a bad bunt attempt right off the grass, but with the time it took for him to turn around and get the ball to Iguchi who was covering 1st, and then from him to Uribe covering 2nd, the Twins' runner on 2nd might have had a small shot of getting back safely if he'd stopped and turned back - yet he's clearly seen trotting past 3rd once he realizes Konerko's caught it and put the play into motion.
I'm not defending LaRoche's lack of hustle, but as the rest of you have stated, his ADD didn't cause that, and IMO he's not the worst offender in MLB when it comes to slacking off by players who either think the play is going to be easy for them (like LaRoche did), or too hard/close to bother (like the Twins runner on 2nd).
LaRoche's mistake was not being aware that Johnson had changed gears. Instead of all the talk on LaRoche, I'd rather hear from Johnson, because I'd love to know if he was being intentionally deceptive from the moment the ball left the bat (and he realized how terrible a hit it was), or if he simply reacted to the opportunity after the fact, once he realized LaRoche wasn't looking or speeding up himself.