He's been our worst D-man so far, but he's also coming off an injury that he shows clear signs of being bothered by. We should be careful about drawing too much conclusions from that.
You can name many others too but, the vast majority of nhl players start peaking around 25/26. You usually won't see a massive improvement after that age range.
Very true, but the majority of cases where you see late improvement it's cases where the player is coming late to the league. There's more to Zaitsev's game than he's shown us so far, and I don't think it's impossible that he'll translate it.
On the larger ice surface he was an incredibly reliable guy, always making the right decisions. With less time so far, it hasn't really been the case. Some, like Babcock, says that the NHL game slows down with experience, and if that happens we might see Zaitsev make better choices with the puck. Another part that hasn't transferred is his shot. He was terrific at finding angles and getting his shot through in KHL. Right now he doesn't seem to find the angles. If that can change, we could see another step.
There's reasons to believe he could be better. It would be wrong to make decisions taking that for granted though.