It's been a long time since I've felt so excited about a (fairly new) shounen action series. (Or, at least, one from a mangaka that I've never heard of before.)
World Trigger begins fairly typically: with monster invasions and an overpowered boy who is more than he appears appearing and saving the day. If I was judging this manga just by its opening chapters, my reactions would definitely be "bored" and "unimpressed." It definitely seems like a "been there done that" series at first glance. And, while it never entirely rises above the story cliches it uses, it does undoubtedly set itself apart with its battles and characters.
Let's start with characters.
I was pleasantly surprised to find that Osamu wasn't a bait-and-switch protagonist. You know the type: appears on the first color page, has a prominent role in the first chapter, but ultimately is shown to have bitten off more than he can chew and is then saved by the real protagonist. Aforementioned boy then either disappears into oblivion or is relegated to the support of comic relief background character for the remainder of the series. Well, Osamu isn't like that. He actually remains an incredibly prominent main character throughout the series, and, as an added bonus, he doesn't get a shounen power-up to make him more in-line with traditional "strong" male protagonists. Instead, even when he is made the "leader" of a team, he remains mostly weak, and his primary strength is his hard-working personality. I don't know about you, but I for one really appreciate protagonists like these who show that you don't have to be superpowered in order to be a hero.
Chika is also a great protagonist to read about, especially because she fulfills all the criteria for the typical damselled girl character (or one who is consistently put into a supporting role), but the mangaka averts this by making a point to say that Chika herself wants to fight on the frontlines. The recent chapters have definitely been great at showing this point.
Yuuma is probably the weakest, character-wise, of the three protagonists, though the mangaka's careful rendering of his backstory (and consequent explanations for his appearance and actions) definitely makes up for it. I also like the fact that, while Yuuma is undoubtedly the strongest of the bunch, he never takes up too much of the spotlight, and the mangaka is really good at dividing up the screentime between the different characters.
Now, the battles are another thing that World Trigger does really well and really differently.
Unlike most other shounen action manga, World Trigger's battle sequences are heavily dependent on strategy. They're very incredibly thought out, too, especially for a weekly series. It's rare that I find a series that's able to put this much thought into action scenes (even monthlies often have trouble with this!) while still having it flow smoothly from chapter to chapter. Honestly, I can't think of another shounen action series that employed this much strategy in each of its battles. The huge cast of supporting characters doesn't make this easy either, but the mangaka handles them all wonderfully.
Of course, the story for World Trigger is in its very early stages right now, and, as I mentioned, the premise itself is still riddled with cliches, so I can easily see this going downhill if the mangaka runs out of steam. However, this really is the first time in a truly long while that I've read a shounen action manga and went, "Wow, this could definitely make something of itself. Something incredible that everyone will remember forever." So I'm definitely giving World Trigger the benefit of the doubt, at least for now.
TLDR; while the story itself isn't all that interesting or unique, the characters and the battles are. read it for them; it's worth it.
edit: OK, almost 100 chapters now, and I'm still really enjoying this. It's still strategic in the best ways and really, really friendly in the way it handles defeats. I myself have gotten really sick of the upsurge in grimdark shounens recently where everything is DEATH and SUFFERING (well more of the latter than the former--shounen leads seem capable of losing so much blood without any lasting consequences), so World Trigger is a really nice change of pace. It's a rare thing to see a manga where the stakes feel real/tangible but side characters aren't dying like every two chapters.
It's still not really elevating itself in terms of overarching plot, but each chapter is so well-thought out and just plain fun that I don't even care. (Also, if you're buying the volumes, Ashihara's comments are hilarious. He's really great at nailing dry humor.) World Trigger really sets itself apart with its consistency--it's just a really solid series with a solid cast.
edit2: Just came to say, now that we're on chapter 226, I'm still enjoying this series a ton. In fact, in between waits for new chapters, I even reread previous chapters just because they're so good. I don't usually like drawn-out tournament or training arcs, but World Trigger's is incredibly fun to read because they rely on super interesting strategic battles in the former and very fascinating looks into each character's own mentality in the latter.
Like, if you're expecting a typical battle shounen, this series is definitely not for you. It has battles, sure, but it's really focused on the strategic mind battle aspect of these encounters rather than the actual pew pew parts. And, yes, it does expect readers to feel some kind of connection to all these various characters, because Ashihara has each of these people's strategies and mentalities down to a T. So if you're not interested in an ensemble cast with relatively equal focus given to other characters alongside our main trio, then World Trigger will probably frustrate you. But, oh gosh, is Ashihara good at making each character distinct or what? All of them have very clearly thought out personalities and reasoning for why they do what they do, and I love it when Ashihara turns the spotlight on a lesser known character and give them their time to shine.
Anyway, yeah, I still love it. It moves through its central plot fairly slowly, I agree, but the stuff we get in between (mainly the strategizing and genuinely interesting mental training tests) definitely make up for it, and they're what makes this series one of my favorites. (It's also a better series to read all at once, then piecemeal, so rereading it has been a blast too.)