Kovalchuk is IMO the most physically insane athlete the NHL has had in the past 2 decades. And that's impressive considering he played in a league that has had some pretty decent athletes. He just never got tired and he could skate like a rocket to boot. You add that to some great hands and a finisher's mentality and you can see why he was so good.
Dude was Ovechkin before Ovie was, he was hyped and he lived up to the billing. He was the forward version of a workhorse defenseman, one of the few guys that played minutes that top pairing defenders played.
Ovechkin created more space for himself with his physicality which also helped open up space for his teammates due to his presence. If Kovalchuk did that, I think he could have been as productive as Ovie, but he'd also probably slow down a lot faster and his career would be shorter than it is. That just wasn't his game. You hear a lot about Ovechkin's drive to score, and while Kovy also was a goalscorer, he didn't play a style that was "all-sacrificing" to get that goal. That's probably the biggest separation for me.