I also was not that impressed witht he Foundation trilogy. After hearing so much about it, I read it about 10 years ago and I was disappointed. For starters, it's really a bunch of stories which almost stand alone, and they are fairly simplistic. The whole concept fo psychohistory was cool, but the end of the trilogy didn't wrap up the predicted future of the galaxy and leaves you hanging. Not having much tolerance for stunts like that, I passed on reading further sequels. Asimov's robot stories were much better.
As for Clarke, I like his short stories better than most of his books. 2010 and Rendezvous with Rama (only the first one) were exceptions, I disliked Childhood's End and, hate to say it, I was let down by 2001, having grown up with the inspiring Kubrick film. Clarke's short stories tend to be more technical and science-related and they get to the point.
One weakness I've noticed in many SF writers is interpersonal relationships among the characters, particularly romantic relationships. They are often forced and awkward, and a novel-length story suffers from this. Perhaps this is because SF writers are nerdy or something. I've also noticed that some writers can't refrain from pushing the edge with sexual relationships as well. Clarke seems to have gone out of his way in 2061 Odyssey Three to include a gay couple, which really had nothing to do with the story, Heinlein got more "free lovey" as he aged, and Larry Niven's Ringworld books regularly feature the main character having relations with aliens. Even the writers of Star Trek regularly push that stuff into their scripts. What's up with that? I like to keep an open mind, but most of that stuff seems to get in the way of the story.