My opinions on this, stated a while back, weren't really expounded upon.
The fact of the matter is, it's because of the series' simplicity that I don't like the books more than anything, and the plot is really rather generic when you get down to the grittiness of it. It was aimed at YA, to be sure, but I've even read YA that was more intellectually engaging than this was.
I tend also to be rather nit-picky about micro elements in stories I read, like the fact that she listed the basilisk as a serpent rather than a lizard. (Off the record, basilisks do actually exist, and they are actually lizards.)
Back to the point above, I really don't care much for simplicity in what I read and that is my own personal taste. Neither, for that matter, do I care for simplicity in the music I listen to or the art I like to view. My favorite music band is not a generic overnight pop icon like Justin Bieber, nor is it an immortally clad headbanger metal band like Metallica. No, my favorites in music are those that can connect with their listeners on a deep emotional level, and those that have little subtleties in both their lyrics and their instrumental performances as to fire my mind off into picking up each individual twist. My favorite in art is not that which would don the cover of a Dr. Seuss book, but rather that with those subtle hints at the artist's thoughts and emotions at one time, that which can be open to interpretation and worth of discussion that can last from twilight until the post-midnight hours.
My taste in books is the same. What I have read as a child may be relatively simple to what I read now, but I felt even those such writings were more challenging and engaging than Harry Potter. And on a more dissective note, I personally found the characters lacked a certain amount of depth, to be honest, at least in the movies, and especially Harry himself, and that the characters all had a typecast to them. Draco, for one. Villainous? Indeed. But he displayed all the emotions of a bad villian without even being human. Lucius was an even worse example of this.
Harry? Harry had to be pushed into the black hole's corner before he ever gained enough human anger to so much as yell at anything. Hermione? Stuck up little snob to be sure, and her arrogance at her intelligence. At least she was shown to be rather more human in that she was in the bathroom all day one day crying over some heartless comments made about her.
Ron I just found annoying. I felt he was perhaps a little too carefree and just didn't seem to have enough outwardly shown ambition to do anything on his own. Always had to stick to Harry like glue.
I do have to admit, I had found the Sorting Hat to be a rather fresh novelty-it really isn't all that often one comes across a talking hat, even in fantasy-but such a minor character certainly wasn't enough for me to fall head over heels for the series.