Mike Trout has improved his defense in center

Every time you think that we've reached "peak Michael Trout," that there's not anything baseball's greatest player could do better, he does something better. A few years ago, there was an argument that he wasn't a true "five-tool player" because he didn't have a strong throwing arm. It got better. Then word came that Trout had a weakness hitting high fastballs. He fixed it. After an 11-steal season in 2015, Trout said he'd set a "personal goal" to improve. He swiped 30 in '16.
You don't get to be the best player alive if you aren't always trying to get better, and this year, his focus was defense. For all the highlight-reel catches, Trout hasn't actually been rated all that highly by defensive metrics over the past few years. It's been the only minor flaw on an otherwise spotless resume.
"There are a lot of defensive metrics out there you want to get better on," said Trout to MLB.com in February.
So far, so good: One-third of the way through the season, Trout's defense looks better. It's been rated better. In an admittedly small sample, Trout looks like he's once again identified something he wants to fix and done exactly that.
There are a few ways to show that. Let's start with Outs Above Average, the Statcast™ range-based metric, which gives or reduces credit based on the difficulty of every catchable opportunity. Trout was seen as slightly negative the past two years, and now he's a plus so far in 2018.
Trout's Outs Above Average
2016: minus-2
2017: minus-3
2018: plus-3
Last year, Trout's minus-3 was 86th of 124 outfielders who had 100 chances. This year, he's tied for 16th of 103 with 50 chances. The range components of other advanced stats largely agree. (By Defensive Runs Saved, he's up from minus-9 to plus-6. Ultimate Zone Rating says he's up from minus-4 to plus-3.) These are huge improvements.
Another way to look at it is simply to ignore difficulty and to focus on the percentage of plays made. Atop the list are Jarrod Dyson, Harrison Bader and Adam Duvall, who have converted 96 percent of their chances. Odubel Herrera is next at 95 percent, then Trout is tied with Chris Taylor and Albert Almora Jr. at 94 percent, just ahead of Byron Buxton. You don't have to love defensive metrics to know that's a group of fielders you'd want to be associated with.
What changed? We wrote last year that part of Trout's issue was simply not receiving tough chances, but that's not really any different this year. Fortunately for us, he made it easy. Trout told us -- in two different ways.
"I tell myself I can catch everything, instead of giving up midway through," Trout told the Orange County Register earlier this year. "It's just a matter of being more aggressive."
We don't yet have a way to measure "being aggressive." But it's interesting Trout said that, because it's exactly what we noticed wasn't happening last year. Just look at this Khris Davis double that would have been a difficult-but-not-impossible catch in 2017, if Trout had only gone for it:

"Little surprised that he stopped at that one," said the Angels' broadcast. "I thought Trout had an opportunity."
When Trout robbed Paulo Orlando on April 13 of this year -- one of the four most difficult catches he's made in the three-plus seasons of Statcast™ -- he showed no such hesitation, laying out to make a difficult catch.

Trout also indicated that he was working on something more tangible. He wanted to be a little quicker getting to the ball.
"What we've been working on is … my pre-set," Trout told MLB Network during Spring Training. "I'm already down when the pitch is out of the hand. I'm trying to get down as soon as contact happens so I can make that first jump, as opposed to sitting there for like a half-second, flat-footed. I've been working on it in the spring."

"It's helped me, because even on foul balls, as soon as the ball gets through the zone, I'm down, and as soon as it's a foul ball, I'm just reacting, as opposed to when the ball is still in the pitcher's hand, I'm just sitting there like this, and then when the pitch is thrown, I'm just a little late. Just little stuff I'm working on."
That was back in March. You can see the effect, somewhat, in a few of the impressive catches Trout has already made this year, and note how the broadcasters describe them.

"What a terrific jump by Trout to haul it in," the announcers mention.

"What a jump by Trout right off the bat," the broadcast echoes.
You'll note that Trout doesn't need to leave his feet, that he makes it look relatively easy. That's partially because he still has elite speed, ranking in the 95th percentile of Sprint Speed, the Statcast™ foot-speed metric. Trout's 29.3 feet per second mark isn't only well above the 27 feet per second Major League average, but is also tied with Dyson and Dee Gordon for 19th of nearly 450 qualifiers. Now, it seems, Trout is taking better jumps to take advantage of that speed. There's an in-progress Statcast™ metric for quick jumps, and while it's still in its infancy, it does appear that Trout has clearly become quicker this year.
This all matters because it obviously helps the Halos, who are in a tight battle for a postseason spot in the American League. But it matters for Trout, too, as he continues to track down all of history's greatest records. For example: As baseball's greatest player having his own greatest season, can Trout put up the greatest season in baseball history?
That mark, according to FanGraphs, is currently held by Babe Ruth in 1923, when he hit .393/.545/.764 with enough defensive value to put up a 15-WAR season. (There have been only four seasons in history where a player has reached 13 WAR, and all were Ruth, between 1920-27.)
Trout, through the first 54 games of the Angels' season, is on pace for 13.1 WAR. It would at least allow him to get into the conversation as having had "the best non-Ruth season ever," which is just an absurd thing to say. That's partially because Trout is the best player in the game, having the best year of his career. (He's currently leading the Majors with 18 homers.)
It's also because defense matters. It's because Trout identified the one minor hole in his game, and resolved to improve it. He did, just like he always does.