Because I liked HP, I thought I'd try some other YA fantasy books, like Narnia (which ok, aimed at even younger readers, but still), and The Book of Three and Dragonborn.
I'm late to the thread, but don't give up on YA. I'm not familiar with Dragonborn, but Narnia and Prydain are usually considered "Intermediate" rather than "YA", so it would make sense if they felt more juvenile to you. Maybe try Lloyd Alexander's "Westmark" series instead... it's more YA and an excellent read.
A more recent YA book I'd recommend for a focus on character and mystery: StarCrossed by Elizabeth Bunce. (Haven't read the sequel, Liar's Moon, yet, but I'm looking forward to it like whoa.
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Other suggestions:
The "Harper Hall" and "Dragonriders of Pern" trilogies by Anne McCaffrey... the first is more YA, but either is great.
The early Xanth books (already mentioned) and also the "Myth-" books by Robert Asprin.
The Book of Words trilogy - J.V. Jones (Been a while since I read them, but it's pinging for me as a good selection for the coming-of-age-y-ness aspect.)
Death of the Necromancer (or really anything) by Martha Wells - she has a real skill with characterization - making characters that jump off the page and make you care about them - and I'm getting a sense that that's what you're looking for.
I haven't read her more recent work, but back when I was first infatuated with David Eddings, I also adored Patricia Wrede, so she might be worth a look.