This is shonen for grownups. Not because it is gory, but because its tone comes off as a much more realistic take on how humans handle crises than most shonen. I like that their manoeuvring gear has limitations, and that it is all too easy for characters to mess up or get eaten; it's a nice change from shonen series where everyone is overpowered and everything goes off flawlessly. This is a series in which we really feel that the main characters are up against the odds, and that really makes the series more addictive.
I feel most people overstate the gore. Yes, there are people being eaten by giants, but this is not particularly graphic (anime being less graphic than the manga, too) and warrants a 'young adult/16+' rating). I'm not a fan of gratuitous gore or suffering, but the series never crossed that line to me. It's less gory than a lot of horror films, but what makes it more affecting is that we lose characters we've gotten to know and care about as the story progresses. And not after long, grand battles: fortunes can reverse in split-seconds in this series. This not only adds realism but keeps the reader on edge; almost anybody in the series can die, and there's always the risk there's a titan behind the next corner or a new plot twist that will jeopardise your favourite characters.
It is also one of the rare series with seriously badass female characters that completely hold their own amongst the boys. Tired of girls who mope in the background in a bikini and are incompetent in fight scenes? Meet some capable women who aren't dressed ridiculously and who aren't there for fanservice! Despite this, I haven't fallen in love with the characters quite as much as I did for series such as FMA, but I still found them engaging and likeable, even when they rely on common shonen tropes. Eren's stock shonen main character 'enthusiasm over brains' approach grates much less than Naruto, and Levi takes the 'sullen antihero' to a new level by literally never smiling. Ever. Yet somehow comes across as less irritating than counterparts like Sasuke. Even Armin, who is permanently crying manages to become an asset to the team. You won't find truly ground-breaking characters here, but you will find plenty to care about.
The main negative would, apart from the slightly depressing realism would be the pacing. Personally, I enjoy more relaxed pacing as it lets us find out more about characters and how they react to what's happening, but I'm aware others may like their shonen more fast-paced. There is an air of mystery around the titans and many of the characters that you will either find deliciously tantalising, or else just plain irritating. If you're impatient this might not be the series for you.
I can only say that you'll have to try it to see if it works for you.
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