I wouldn't say it was rare to see him shut down elite players, as he was very effective against Crosby in his rookie campaign in the playoffs.
Like you said he has always been reliable on defense, but did make "mistakes", especially when starting the rush, trying to catch the other team off guard, which sometimes backfired. Since he was/is still young, it was expected to see him blunder. This year he seems ready to be "the piece" that we need on D. Yes he isn't perfect, but he is very effective. He is taking a lot less penalties, which helps not only the team, but his image. I always found he was getting the short end of the stick when some of the calls made against him were really borderline.
Where I feel he has made the most is his ability to imitate and grow. When you watch Subban on the PP, he plays very similar to Markov. He no longer rushes or gets rid of the puck quickly to avoid a turnover. He reads the ice well and knows how to protect the puck without much chance of a turnover. Even at ES, you can see that he isn't trying to rush up the ice the moment he gets the pass, he reads the ice, watches what is developing and is ready to skate or make a first pass, which he also improved on.
He is finally able to learn and adapt his game to the style on the ice. This was something I didn't like, but knew it was a matter of time that he would mature. We always knew he had the raw talent and the desire to win, but his ability to learn was always something that was questionable. The way I see it is that I compare to myself at university. When I first started I thought everything was going to be easy and then you start struggling, you either try and force it or you give up. As you experience more, and you tend to face new challenges that you struggle with, you have to ready to find a solution. We all saw that Subban had a stubbornness about him, not necessarily negative but when you are talented at something and you are used to winning, you get complacent. We saw that a bit under Martin. This year he seems to be ready and open minded about everything. He no longer seems complacent and is always ready to improve.
I always liked PK since I found out more about him after we drafted him. I understood that MB wanted to give him a short term deal, but I was one to say give him a long term deal, because he is for real. I hope this doesn't come back to bite us when he has a new deal to sign.