Mild spoilers:
Disagree completely with some of the people in here (and the widespread acclaim the film has garnered also disagrees). There haven't been many action films as of late that attempt to strip the genre bare, taking the "less is more" approach to story and wrapping it in some of the best action choreography to come along since The Raid. I'm sick and tired of CGI, and maybe that's your thing, but this is a film that gives it's star a chance to do real stunt work. It's an homage to the action films of old; an exercise in craft and aesthetic. Some people are also not giving it due credit for the world building it establishes in the "noir graphic novel" style.
Let's start with the emotional core of the story; love and loss. John Wick is a man whose past is catching up to him in the most profound way possible. No, there are no "bad men" out to get him for something that he did years ago. His wife dies, quite tragically from an undetermined illness, in an emotionally effective opening that immediately helps to make John an identifiable and sympathetic character.
Circumstance forces John's hand. His only outlet for his grief is violently ripped away. You give the character an emotional anchor, you mess with it, and you set him along that path. You watch him, conflicted, as he trudges back into a world that knows very well who he is and what he's capable of. Do we need endless exposition and backstory? Do we need to be force-fed plot because the movie thinks we're stupid and we need our hands held through the whole thing? How many action blockbuster films do that nowadays? All of the Marvel films, for sure.
John interacts with characters and you immediately get a sense of the history of that world, the characters at play. Chop it up to cosmic fate intervening or whatever you want to call it, but his is an assassin's world that is populated with characters who don't need to say much to acknowledge each other.
The action itself, on the other hand, tells it's own story. The steady cam shots, the long takes (none of this quick cutaway, shaky cam bullshit), the practical effects, the fight choreography with the central axis relock movement, the lighting and music in the Red Circle night club scene...all of it is so fucking well done. All of it comes together to show how John Wick is an unstoppable force that has been set loose against the immovable object he himself helped to create.
John Wick is more than a film about revenge. It's a film about fate. If you buy into fate, you accept that John's life was always going to lead down this path. He tried everything he could to make his own fate and choose another path, but killing is an ugly business that nobody can walk away clean from. It tears at the fabric of your humanity. It's all there in the conversation between him and Viggo.