Admittedly, some of the minor actors (especially some of the children and teenagers) aren't great. But the principal performers are wonderful, and the filmmaking transforms it into something special, something different.
Typically, a film will shoot for an intensive period of weeks or months, all at once, so the cast and crew can move on to the next job. Nearly all films are shot out of order—you might shoot your last scene before your first, if the schedule works out that way. And if you to portray the same character at very different ages, you have a few choices: can do it with makeup (often with very creepy results), with CGI (think Benjamin Button, I guess), or with different actors altogether (my favorite being Kirsten Dunst and Samantha Mathis in the 1994 Little Women).
But because of the way Boyhood was shot, the same actors portray the same characters throughout the entire film. So we see them grow up and grow older as we're watching the characters grow up. We watch children go through their awkward stages as they become teenagers and then young adults. Music plays a big part in this film, but when you remember it's just the music that was actually on the radio when they shot the scene, it feels less like planned, mannered set dressing and more like actual life.
Richard Linklater likes to mess with time. His work includes movies as diverse as Slacker, A Scanner Darkly, and School of Rock, but he's perhaps best known lately for the "Before" trilogy—Before Sunrise, Before Sunset, and Before Midnight. Those films, we discovered when Midnight was released last year, were all shot nine years apart and portray the relationship of two people (played by Hawke and Julie Delpy) in, yes, nine-year-apart installments. The films remind you what it's like to fall in love, to have conversations that seem to last a lifetime, to weather the difficulties of relationship, and to wonder whether relationships are even worth the trouble. (Read Ken Morefield's excellent take on the trilogy from last year.)
Boyhood, too, is interested in how relationships evolve over time: in this case, parents and siblings, step-siblings and cousins. Our relationships to each other in families don't stay the same over the course of our lives. Siblings start as our playmates, then our rivals, and eventually become our friends. Parents go from being protectors to, sometimes, needing to be protected—or they do some growing up of their own. And of course, we rarely notice ourselves changing. But imagine: what if you could watch yourself growing up on screen?