Premise in 30-words-or-less: a boy is being raised/trained by a trio of aliens who are working to clean up the mess their own people made thousands of years ago.
The most surprising thing about Steven Universe is that what looks like a simple Monster of the Week show with a young POV character is actually an extremely mature, plot-driven sci-fi story. Everything the show refers to as magic is only magic in the Clarke's Law use of the term: the aliens are actually hard-light constructs and all of the weird monsters and dungeons and stuff are left-overs from an interstellar empire's attempt to harvest the earth thousands of years ago. The plot in a nutshell is that one of the aliens and her followers led a bloody, desperate rebellion against her own people to save the earth from being denuded and the native life from being killed off. After the rebellion, the leader and her tiny handful of surviving followers worked to catch all the bits of alien tech that were still rampaging around while also being on the look out for any attempt by the empire to reclaim the earth. Eventually this leader falls in love with a human and effectively dies in childbirth (her equivalent of a light bee gets absorbed into her son, which is why he has a gemstone for a navel), leaving her followers reeling. Steven gets to grow up raised by a trio of warriors who have never been a human nor a child, and have many complicated feelings towards the boy that kinda-sorta killed their former leader and kinda-sorta is their former leader.
The thing is, the POV character is a pre-teen and walking embodiment of a Friendship Speech who has no idea how to fight or use his powers (think Aang if he started the show as a neophyte air bender). That combined with every episode being just 11 minutes long means all of these mature themes, high-end sci-fi, and plot elements are hints and suggestions that build over time; SU has a big story to tell but it tells it one slice-of-life at a time. Steven matures and the stakes are raised and the story becomes clearer over time, but this show lets everything unfold bit-by-bit. This sort of patient story-telling is a rarity, especially in the medium of kids shows. Of the three cartoons I'm posting about here, Steven Universe was the show I had the most difficulty getting into but it's also the one I've gotten into the most. It's like looking at a Magic Eye picture: early on you just see a jumble of childish action but after a little while the show comes into focus and the result is impressive.
Lastly, I'll say this cartoon has some pretty radical ideas on display for a kids show. Steven is wholly lacking in machismo, his father is only a supporting character who is in no way a tough guy, and all of the other important characters, heroes and villians alike, are female. Steven is raised by three "moms" so the topic of unconventional family dynamics is front and center.
Also, there's a lot of sex. And lesbianism.
Really.
It's hidden behind a metaphor but it's a damn thin one. The show is rife with suggestive elements and mature themes and it's glorious.
You really should watch Steven Universe.
Episodes
Episodes
Episodes
EDIT: Copied the following from further in the thread
Alright, let's talk a bit about getting into Steven Universe. As I mentioned in my initial post, it was actually kind of hard for me to do, and reading the thread it looks like others have been on the fence for various reasons.
So, think back to book 5 or so of the Harry Potter series. Things are getting increasingly dire for Team Good Guys and Harry is becoming a full-on teen, with all the angst and bitching that entails. Remember how most people, possibly including you, found those sections to be annoying? Well, the fact is the Harry Potter series is better as a whole because those sections exist. Teenagers are whiny bitches to one degree or another and Harry had been through more than enough traumatic shit to deserve some on-camera angst. If he'd been written without it, his character and his story would feel flatter and less realistic. That doesn't necessarily make reading those sections any more enjoyable but the improved verisimilitude is worth it.
Those first 6 or so episodes of Steven Universe are like the angsty parts of Harry Potter: necessary but not to everyone's taste. At the very beginning, Steven is at his most naive and least empowered. At the very beginning, the setting (with all of its hidden depths) is at its least defined. At the very beginning, the plot is more-or-less idling in "monster of the week"-mode. And at eleven minutes each, that means sitting through just over an hour of material before the show starts to get good. I can certainly see people being unwilling, or at least hesitant, to jump in to that.
However, I will say that it is absolutely worth the effort. The show is, no pun intended, a real gem and more than makes up for its slow start. Episodes like Jail Break and Sworn to the Sword represent some of the best entertainment I've ever seen packed in to an 11 minute window.
My advice for folks on the fence is to watch the first episode and then watch the "Connie" episodes (scroll down to Episode Appearances section) since those are consistently high quality and because Connie is a great character. Keep watching those until you get curious enough to go back and watch the skipped episodes or decide definitively that this show just isn't for you. That way, by the time you've watched an hour of television, you've seen some choice episodes and had a taste of the plot, setting, and character development you can expect.