Well, obviously I was going to get Greymane for Heroes of the Storm. He's a Worgen, and uses that as a core part of his gameplay.
Greymane is a hybrid assassin, functioning both at range and in melee. You switch between these by switching between the human form (armed with a pistol and a sword) and Worgen form (claws.) Each of your abilities changes when you switch forms, except heroics, one of which always puts you in human form and the other always in Worgen.
While in Worgen form, you do extra damage with basic attacks.
Much like the Butcher, Greymane might fool you into thinking he's tankier than he actually is while in Worgen form. Essentially, his melee form is more of a high-risk, high reward kind of state, as he doesn't have the survivability of a warrior, but you'll be able to put out some really nasty damage.
Q: In human form, this is Gilnean Cocktail. You throw a bottle (basically a molotov cocktail) as a skill shot. If the bottle hits a target, it will deal light damage but then deal higher damage in a cone to enemies behind the target
In worgen form, this is Razor Swipe, which sends you forward a very short distance and deals damage to all enemies around you.
W: Inner Beast, which is used in both forms. This increases your attack speed by 50% for 3 seconds, but each basic attack will refresh the duration. You'll want to use this as much as possible, but especially when you're beginning a fight that you expect to last a while, as you'll be able to keep this refreshed as long as you can keep attacking.
E: This lets you switch between forms, but also works as a movement ability.
In human form, it's Darkflight, which sends you at the target and transforms you into a Worgen while dealing damage to the target.
In worgen form, it's Disengage, which sends you in the direction you aim it and then transforms you into a human.
Go For the Throat has you leap at a target and deal a large amount of damage to it. If you kill a hero with this, you will be able to use the ability again within the next ten seconds without worrying about the cooldown.
Marked for the Kill starts has you shoot the target with your pistol, marking them and making them vulnerable for 5 seconds. While the mark is up, you can hit it again to transform into a Worgen and leap at the target to deal more damage to it.
The obvious strength to Greymane is his versatility (and the awesomeness of being a werewolf.) In the very little experience I've had with the character, I tend to stay in human form most of the time, but when I have the advantage, I can shift into worgen form and really devastate things. Still, just having to be in melee range makes you vulnerable, so I recommend only going worgen when you're ready for a takedown. Luckily, Disengage is a great escape move.
Inner Beast is pretty easy to maintain if you can stand still and attack, and the cooldown's short enough that I'd say in most cases you just want to pop it once you get into a fight. But it's only basic attacks that refresh it, so you'll definitely want to try using it when you're able to stand still for a bit.
Worgen form inherently increases your basic attacks by a significant 40%, so if you're attacking buildings without much need to dodge enemy attacks, I recommend going Worgen to take them down that much quicker.