This A.O. Scott review sums it up well. Scott identifies "longing" as the central theme of the movie, which I think is spot-on.
What [Director David Robert] Mitchell gets splendidly right in this quiet, observant film, is the unsteady mixture of sophistication and naïveté that is central to the modern American teenage way of being in the world. These children — the oldest character is home from college, and there is not a parent in sight — hardly know what, or who, they are supposed to want, but yearning seems to be both their birthright and their responsibility.I think MotAS nails the fun-mixed-with-sadness feeling that permeates many adolescent nights.
One big question I did have while watching this is when it is supposed to occur. The idea of "sleepovers" seems somewhat antiquated already. The clothes and grooming are modern, but the cars and many props are indistinctly old, and there's not a cellphone in sight. Did this happen a long time ago, or has the present not yet arrived in this sleepy Michigan suburb?
I'd guess this vagueness is mostly on purposes, because it allows people of pretty much any age to latch on to nostalgia by setting the movie to no more specific a date than "sometime in the last 25 years."
Edited to add — I forgot to mention that the acting is a little rough. This didn't bother me, because these characters are supposed to be awkward. Awkwardly delivered lines reinforce the theme of the movie, rather than detracting from it.