Dungeon ni Deai o Motomeru no wa Machigatte Iru Darou ka is clearly quite a mouthful to say, but if there's one thing that bothers me, it's that the title's somewhat of a lie. Our ultimate shota protagonist Bell-kun carries the very generic personality of "an innocent guy with a big heart," and as he has a crush on this one, very strong female adventurer, he's (as usual) unable to see eye-to-eye with all the other girls that've fallen for him, almost each and every one of them practically ready to carry his offspring at a moments notice. So basically, the series is following the usual bullcrap seen in a lot of harem manga. I don't have an issue with every harem series out there, but it's kinda annoying to see Bell, once again, not even TRY to "pick up girls in the dungeon." He always gets flustered + indecisive when the ladies make their advances on him, and even when he awakens to find himself alone with his crush (who saved his ass a second time), he runs off again out of shear embarrassment. So in terms of the series's (incredibly one-sided) romance, it gets somewhat annoying after a while.
The series offers some other elements as well, including action and fantasy world-building, yet not even those parts are very well defined. The action comes off as pretty minimal, with not a huge focus on the fight scenes presented. Maybe with one monster so far they prolonged the fight over a couple chapters, but most of the time, the fights are wrapped up as quickly as possible, with one-shot moves from our thief/rogue/assassin protagonist + adventurer/knight/paladin-thing heroine. I can understand that the artist (probably) has a stronger background in illustrating rom-com harem stories compared to action-based stories, so I don't "expect it" to have the artistic quality of fantasy series like Berserk and Gate - Jietai Kare no Chi nite, Kaku Tatakeri. However, when it comes to fighting being an incredibly important element to Dungeon's core story, it's kind of lacking in quality so say the least...
The fantasy world-building is directed at readers with a background in playing/knowing the different elements of fantasy RPGs. Characters essentially leveling up and having corresponding numbers to their various stats, using grimoires(?) to learn magic spells without having to really read them at all, potions for healing, etc. etc. The idea's not too bad, but the world-building is very slow, with all attention directed at Bell-kun interacting with the individual female characters.
Oh, and so far, there isn't much of a plot presented. Bell wants to become stronger, and so thanks to the magical powers of a middle school crush (no joke, his crush on a girl is enabling him to level up so much faster than any other character), we follow his journey to become strong enough to impress his crush. As he (barely) works hard to become much stronger, he never considers to try and "man up a bit" during this time, thus leading to his embarrassment and indecisiveness with the ladies at every turn. Not much else can be said for specific events throughout the story, as they either make Bell stronger or introduce another girl to his harem, and as rennov stated, they'll fall for him based off the simplest of triggers.
I would even bother to argue that Bell's somewhat selfish, as he understands that he's the only member to his Loli-Goddess's familia and can die in the dungeon, leaving her all alone without anyone else to rely on. I really thought that he'd try to recruit some other people, working their way up as a small and poor familia on their way to challenging other big shot familias. Maybe the Light Novel does a good job with that, but in this series, Bell never bothers to think about that, and always must focus on becoming stronger for the sake of his crush... If it's gotten annoying for you, how I constantly restate Bell's goal, you now may know how I feel with series beating me over the head with it...
I haven't read this one yet, so if you haven't gotten the chance to try it out either, hopefully we'd both be very content with what it has to offer in its dungeon-based fantasy world: Dungeon Meshi
From what I heard, it's very funny, and creative for what it does with their characters' dungeon adventure(s).