So a thing about most adaptations of Sherlock Holmes is that they go one of two ways. Either they set something historical, with historical vibes. Or they set something contemporary, with modern vibes. And those vibes almost always translate to the actors playing Holmes and Watson. Sometimes you'll see a very modern seeming Holmes in a historical adaptation in order to feel intentionally out of place, out of time, and add to the mysteriousness.
Watson is the first time that I can think of that someone set the adaptation in the present day, made most of it feel very modern, and then cast a main character to feel very historical. He's classically handsome with a stage actor's build, he has a wardrobe designed to look 'timeless' or even old fashioned, and many of his interests are antique in a way that would usually either be explained or limited. The actor even uses speech patterns and body language that wouldn't feel out of place in a historical adaptation - but not so much they feel out of place in the modern one, either.
I just enjoy the vibes of this, is the basic thing. It's unusual and that makes it feel a little unique, which is important with how many adaptations there are out there. Usually we'd be seeing this kind of character act old-fashioned in a way that was intentionally isolated and offputting, but here it's pretty subtle. And it really helps it feel like an actual adaptation - it's canon compliant in a way.
But it's also fascinating from the perspective of when the show is set. It's directly post Reichenbach Falls, with no indication so far of whether Holmes is even supposed to be dead or alive. It's irrelevant to the show, for the most part. So of course we need to see Watson's life changing; he has to build a whole new life that doesn't have Holmes in it. It has to feel different. So having him be the one character that feels subtly out of place? Outstanding.
But more than that, there's a kind distance created by the time difference, almost as if Holmes died in the historical setting, and Watson lived into the new one. An acknowledgement to us that most or all of the stories are canon to the show, while not being strictly relevant to the episodes we're going to see. Also, though, a way of showing that Watson is moving on and building a future. He's finding a way to move past something that didn't just happened, but rather happened some time ago now. It's a show about long term recovery rather than immediate grief.
From the metafictional perspective, it's also interesting commentary on taking a well known (and beloved!) character, and creating an entire new world around them. While we do also see Mary frequently, most of this world is built just for the show. He has an interesting cast of interns, and a clinic built just for him, and a life in a city that never had to do with Sherlock Holmes. But he feels a little out of time because he's someone else's character being gently situated in this brand new world, delicately and while cherishing the original medium. It's pretty fannish, in that way.
There's so much room for signaling going forward, too. Maybe he'll develop a more modern style as metonymy for moving on. Maybe he'll cling to his memories of Holmes, but only in certain situations, or maybe he'll be more obvious about some historical thing. Maybe the other characters will develop slightly more classic styles to indicate that Watson has opened up to them and started telling some stories, even if they're personal.
They're going somewhere very cool with this, even if it only ends up being a small part of the story being told.