Based on what I've watched and looking at some of his stats, I think DeAngelo is probably an NHL defenseman, but not a very good one. I'd keep running him out there, its not like we need to win or the other defensemen are any better.
He's pretty good offensively, although not great offensively. We've gotten a decent enough sample of his offense at the NHL level. He's played 55 games, and with about average #6D minutes, so while we'd like more data, we have enough. He's put up what equates to about 25 points over 82 games with 6D minutes, which is reasonably good for the role you'd expect him to play. He's probably capable of a little better in that role as he improves, maybe like 30 points in that role, but his P/60 over the two seasons in all situations is around mid 50's out of about 180, so he's pretty decent at scoring points, which is his calling card.
Unfortunately, in my opinion, the defense is just not very good. Its passable on a bottom pair with sheltered minutes, its not like ECHL level, but I don't see much upside there to be more than bottom third in defending among NHL defensemen. He's small, weak, bad in board battles, bad in front of his net and makes some very risky decisions defensively. He takes away a lot of his offensive contributions by not being very good defensively. Shatty does the same thing, but he's elite offensively. DeAngelo is not elite offensively. I guess he could become elite offensively, the raw offensive skills are there. He's not shown elite offensive play yet though, although he has shown pretty good offensive play, so maybe with another few years, he could get there.
If he doesn't show elite offensive play eventually, I see his role as a #6D, maybe a #5D, who plays the PP, gets sheltered minutes, puts up points, but defends poorly. Thats a useful player and he's still young so some minor improvements should be expected and he has talent to make major improvements. He's also on an ELC, so that works in his favor as well, but I'm not really expecting that much more than that from DeAngelo than what we are seeing right now. I don't think he's going to become Karlsson just because he's young and flashy.