As if by magic, at the mention of Greymane I am summoned. Admittedly late, but who cares.
When he first came out he wasn't very good, and no-one really made much progress with him. Wolf form was very weak due to the vulnerability, and cocktail was...cocktail. He got buffed not that much later, and suddenly exploded in Europe and Korea. People figured out the best build with cocktails, and he became a really powerful ranged assassin with an execute. Cocktail allowed you some amazing poke damage, and then when necessary you could just delete someone. He was a superb ranged based assassin, and fitted in any team as a ranged assassin. This is when NA was going "huh? On the podcasts and describing him as a jack of all trades, whilst Europe was going ham on him because of his ability to do great ranged damage and delete people. NA hadn't figured out that whilst he is very vulnerable in wolf form, if you just go cocktails and only use wolf form when you can guarantee the kill he is an absolute monster and a very viable backline threat.
The nerfs to Greymane changed that - without the cocktail talents, he simply doesn't have the poke anymore. Without the poke, he couldn't fit into the standard range slot at all, so usage heavily dropped off to the point where he wasn't considered viable.
What Korea then figured out was that if you go all in on supporting him (Uther, Tyrael etc), his wolf form auto attack damage is so ridiculously high that he can carry a team regardless of his lack of defences. The changes to executioner pushed him over the edge - he hits for over 600 damage a swing, at 1.5 swings a second, which is easily the highest auto attack damage in the game. That's why he has come back so strong.
But it's a completely different way of playing him, and a different team build, than he had originally when I was raving about him. He's not a standard ranged assassin anymore, he *has* to be supported heavily to be effective. But Christ is he strong - I would not at all be surprised if they removed executioner from him or gave him a weaker version.