I think I've been over this ground before, but I'm going to give a brief synopsis of how I perceive Adam Larsson's career as a Devil has gone - I will consult numbers for this, but they won't really augment my points that much, I don't think.
Year 1 (2011-12): I can't remember who Larsson's first regular D partner was, but stats.hockeyanalysis tells me that Larsson's most common D partner was Bryce Salvador, and that sounds right. The lineup was something like Tallinder-Fayne, Greene-Volchenkov, Salvador-Larsson. Larsson struggled mightily on the power play in Year 1 even though he was the main QB - his shot differential was only 31 shots/60 minutes, which is pretty darn low, and the Devils gave up a lot of shorthanded goals. His issue was chiefly slow decision-making - he wasn't sure what to do with the puck when he got it, and often thought he had more time than he did. In addition, Larsson liked to hold on to the puck for as long as possible to see if a pass would develop. Compounding this was his slow acceleration - he'd never catch up if he lost the puck. in short, he wasn't ready for that role and finally when Kurtis Foster arrived he was taken off the power play. Still, despite skating issues, he was an effective defenseman and produced impressive breakout passes. He was gaining confidence and playing reasonably well until he was hurt in February - he comes back and is given less ice time and doesn't appear to be making great decisions. He's a healthy scratch through the first round of the playoffs, but gets in there against Philadelphia and looks quite good. Then he's taken out against the Rangers and never gets back in. Still, a good first year all in all - he broke even as a 19 year old NHL rookie, and that's really quite rare.
Year 2 (2012-13): Larsson posts 19 points in 33 AHL games as the lockout eats up the beginning of the NHL season. Even so, he's left out of the NHL rotation as the Devils go with 8 defensemen - he doesn't get into game action until Game 6. Nothing really stands out to me about this year at all - he was mostly paired with Andy Greene and really did not look impressive - the breakout passes were there, but the rest of his game wasn't. He struggled with gap control, he was slow to cover passes by the opposition, and of course he was being beaten wide far too often. I think this year is when I noticed that Larsson takes a lot of hits in the D zone, more than his share.
Year 3 (2013-14): Larsson begins the season in the lineup. He plays the first 2 games and is a -4. He stays in for another game where he's even and is then a healthy scratch. I think this is where DeBoer makes his comments about Larsson not being a -2 every game or whatever. The issue isn't just that he's a -4, it's that he's being beaten wide in the same fashion as he had been last year. There appeared to be no improvement in his skating. Gelinas gets called up to the team in October and Larsson is his D partner early on and that D pairing does quite well - they get very well protected zone start wise but they're still doing quite good territorially. Then Larsson gets hurt, goes down to the minors, and returns briefly at the end of the year. Everything seems to be status quo. His territorial numbers for the year look good but they're in a pretty small sample.
Year 4 (2014-15): Larsson doesn't look particularly good in preseason, to my eyes. He's scratched to begin the season and talks about how it's the most difficult thing that's happened to him in his career so far. Trade rumors begin to form around the league about him - Garroich writes that piece about Larsson maybe being a trade piece but teams are wary because of his slow decision making. When he finally gets into the lineup though, things appear different. His breakout passes aren't as special, but he's no longer holding on to the puck as long - he's using the glass and the boards occasionally. He's not taking huge hits seemingly every game. His gap control isn't horrible, he's not getting turned around as often, and he really looks strong on the boards. In addition, in the offensive zone he's not holding the puck looking for an open stick - the 'deflectable shot' that everyone talked about is gone. Adam's just getting the puck and ripping it when he has a chance, generally. It's again a small sample but the numbers are telling an interesting story for Adam this year - he's breaking even despite very difficult assignments. He broke even in 2012-13 too, except that team was much better than this team - his Corsi Rel is positive this year, way negative that year. And I haven't talked about his PK work which is generally very strong.
We'll see how the offense develops - recent numbers notwithstanding, I don't think he's all of the sudden going to turn into a dynamo there, but he didn't look awful when put on the power play recently, and I think with his speed he'll always be prone to short handed breakaways against if he does play there regularly. He's still developed into a formidable player right when NJ needs it - because for as good as Larsson and Severson have looked, Merrill and Gelinas have not.