Subban doesn't make it for me, and I'm pretty well known as more of a "peak" guy. If a defenseman is going to go to the Hall, one of the first boxes one looks to check off is his number of Norrises - he should have at least one. Okay, he's got one. Check. After that it falls apart rather quickly.
Thing is, even when he won his Norris, it didn't feel to me like he was the best defenseman in the league. He was a high scoring defenseman, sure, but that really doesn't make you the best overall. Another thing that supports this is has lack of inclusion on the Team Canada roster. When a guy is truly the best in the NHL, he should be a no-brainer for a national team, even one as strong as Canada's; in fact you should be on the top pairing. It's indicative of hockey minds thinking that he doesn't "give you the best chance to win". And it's as simple as that.
Another thing that one has to wonder about is why Montreal traded him during his prime. And then Nashville does the same.
His career is not over yet, and he's still young enough to do something special given the right circumstances. But, if he remains on his current trajectory, he's not going to make it.