Poor writing was my first and foremost gripe: obvious character motives and an almost total lack of surprising plot twists. To be fair, I also re-read The Star Diaries which I first read (and loved!) in my early teens and I had the same issues with the author's workshop. That being said I think Asimov's prose hasn't aged very well in the light of actual technological progress (information storage and retrieval) and you just keep smiling while reading about Encyclopedia Galactica.
I read it as an adult and was disappointed by the flatness of the characters and the almost-total absence of any female characters.
To clarify: I'm not saying that people should shoehorn women into stories to satisfy a quota, I'm saying that stories that include women are simply more interesting, because they display a wider range of humanity and of the human experience.
Or, to put it another way, it's just boring to read a story where all the characters fit within a very narrow range of personality.
Sure, but 2500 years passed since and you'd expect writing to get a bit better. And yet Homer's characters are way more nuanced than Asimov's. Also, in Plato's Dialogs characters are completely irrelevant and are merely instruments that directly represent philosophical standpoints.