There’s a video by Digibrony called “Embracing Girliness” going around, and I think it does a good job highlighting how MLP criticizes one of the fundamental injustices of our society, that “feminine” traits are devalued relative to “masculine” traits.
However, I think he still implicitly assumes that there are such things as objectively masculine and feminine traits, and as such misses one of the other things the show does well, which is exploding the masculinity/femininity binary to begin with.
While male and female humans differ physically, the notion that certain psychological traits or interests are “masculine” and certain “feminine”–a belief known as gender essentialism–has no basis in reality. This is not (just) my typically liberal, feminist opinion; it’s a matter of scientific fact. (For the less statistics-minded among you: If people fall into two distinct groups, then there should be two spikes on the graph, one for each group, as in the strength graph behind the link. If there aren’t two spikes, it means that people don’t fall into two distinct groups, as in the assertiveness graph. In the latter case, it possible that the “average” man is more assertive than the “average” woman or vice versa; however, there’s so much overlap between the two groups that the average is basically meaningless.)
Society has arbitrarily decided that men should have certain traits and women should have certain other traits, and it punishes people who do not possess their gender’s assigned traits or, worse still, possess the other gender’s assigned traits. However, that’s something we collectively decided to do; we can just as easily collectively decide not to do it. The “just like a boy” scene Digibrony shows is a rare misstep in a show that otherwise does an excellent job of avoiding gender essentialism by giving every main character, Spike included, both stereotypically masculine and stereotypically feminine traits.