Sledging is, or was, meant to be about putting some doubt in your opponents' mind.
In contact sports that's been around for decades if not more, one-upmanship on your opposite number. The Aussies and Windies did a great job of bringing that into cricket.
But over the years it's changed, into frankly just abuse. Calling someone a prick isn't sledging. Neither are threats. And a send off definitely isn't. Id bet those guys giving a send off in the cricket wouldn't be so brave to make similar comments after a try in a game of rugby, with still time in the game for physical retaliation...
Sledging in the traditional sense, part of almost any game. Sledging in the context of what it's become - led by the Aussies and South Africans but exacerbated by every nation over the last few years, is a disgrace. And I think the reason NZ are being praised is that they are taking a bit of a moral stance over it, and looking to move away from the modern version of it.