Something new, not really comparable to the classic American musicals

Rating: 9/10 (13 March 2017)

Aside from being one of the most genuinely enjoyable movies I've seen in years, La La Land is a very interesting film without any direct peers (although I suppose, in a way, it is "Whiplash After Anger Management"; it will be interesting to see if Damien Chazelle has another story in him). As others have pointed out, it's more like Jacques Demy's musicals than anything else, but it's really not much like those either and nobody under around 50 (aside from dedicated musical freaks and faithful attendees at repertory movie houses) has seen Demy anyway.
You can't compare La La Land to any previous American musical romance, not directly (the few people I've talked to who say they didn't enjoy the film insist on doing exactly that). What Chazelle has done is tweak just about every element of the classic formula for the modern audience, then integrate those elements into a very modern, very emotional storytelling form. In doing this, he's actually created something entirely new. It doesn't matter at all that his leads aren't particularly great singers or dancers and it doesn't matter at all that the score is a little thin; it's the total package that has made this movie such a stunning success with audiences. It's modern cinematic alchemy which draws on the past glories of the American musical without actually copying them. As a film it completely avoids nostalgia, even as its lead characters drown in it at times. That's quite a trick to pull off, just as it's a real achievement to employ magical elements like "dancing in the stars" with a modern audience and not get laughs where you don't want them.
By current standards, virtually all classic American movie musicals are defined by their episodic structure. About a month before I saw La La Land, I saw the TCM revival of Singin' in the Rain, which is probably the consensus choice as the most enjoyable musical of the classic era. It's also one of the very best written, but it's still a collection of outstanding production numbers linked by a plot that is almost incidental. That's perfectly fine and it's what we expect from the classic musicals; it's part of what makes them so much fun.
La La Land is a different kind of fun. It's all about its own structure; it has a story-driven flow to it and is far more dependent on strong acting than the old musicals ever were. It's not just a matter of removing mid-century corn from the script; it's built around a very modern form of storytelling that was just starting to emerge around the time musicals went out of fashion. It is much more emotional than it is technical (in its music and dance as much as in its words) and it wouldn't work at all without top-flight acting, something you definitely can't say about even the greatest classic musicals.
Since it's unlikely we are on the cusp of a glorious new age of musicals, La La Land will likely stand as a singular achievement: maybe the most emotionally realistic musical ever, while still a total escapist fantasy. I find that very often the best films are the ones that had the least chance of getting made; it's a perfect example of one.