Lou singlehandedly kept the Devils in New Jersey by keeping them a perennial contender in the early 90s and getting them a Cup, followed by two more. For this, we'll always be thankful for him.
He also did great damage to the team after the lockout by awful drafting, bad development, and moves meant to keep open a contending window that really should have been closed after the 2012 Cup run. Right now with him, it's the same story, but on Long Island instead of New Jersey. It's clear that in Toronto, he wasn't in full control, and Leafs fans should be thankful for that, because otherwise it'd be the same story.
His entire legacy will be mixed. You can't take away the good years but he's been bad at his job for at least a decade.