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.