I split this document off the note I was writing on Iceworld. While I've read only 3 of his stories yet, I probably will end up reading everything by him. He is that good, if you have a certain kind of taste.
I can see some common themes, & some care to take to appreciate his stories. I am putting them here so I can link from other documents. Will edit it as necessary, if rest of the stories substantially change these assumptions.
On "troubles".In one of the interviews somewhere, he mentions an essential plot element of his stories - "troubles". How can there be a story unless there is trouble? Protagonists take actions to overcome these troubles. Might apply to some other authors too.
Point of view.A key element of his stories is to challenge our most basic assumptions about the world around, & about the experience we have gathered living & the instinct have gathered evolving. So changing the world in a fundamental way is central to plot - massive gravity & gravity gradient as you travel on the planetary surface in "Mission of Gravity"; a planet so hot metals are usually liquid & substantial day/night temperature difference in "Uncommon Sense"; a planet so cold even the "air" you breath is solid in "Iceworld".
Figuring out these worlds nearly always requires going back to basic physics & chemistry - key components of his stories. You find extensive illustrations of what is called experimental method in physical sciences. If these subjects bore you, he is not likely to be your kind of author.
"Human" interest elements.Plot elements involving the protagonists - human or alien - tend to be secondary. They exist so there is a story.
So you need to recover a treasure lost or jump start on wonderful technology in "Mission of Gravity"; you need to save your life from assassins in "Uncommon Sense"; you need to hunt down drug runners or make a bargain dealing with crazy traders in "Iceworld". If these themes are your primary interest, you would be better off with some author other then Clement. Don't touch a Clement story unless you can live with large parts dealing with fundamental sciences.
Aliens.When I began this site, I had a rather novice interpretation of aliens & sometimes used to hate a story because aliens don't behave so different from humans. 300 stories later, I've grown up a bit.
Aliens in Clement stories behave a lot like humans. They are different from us only in having learned to live in a different kind of physical environment; even there, only one or two aspects of this difference are stressed. They are essentially human - only got slightly different biases about the physical environment, & reflexes & fears due to this.
When he cooks up intelligent life at all kinds of weird places, I now no longer interpret it as any optimism that life is everywhere - though no one knows for sure. What he is probably telling is: assume there is life there; what kind of life would make sense there? So you have ground hugging beings in "Mission of Gravity"; liquid metals serving as "blood" in "Uncommon Sense"; sulphuric "air" in "Iceworld".
Since these aliens per se are not central to the plot (the weird world is), they are not really developed in detail. Only a couple of key elements that are dictated by the world in the story.
Aging of stories.The ones I've read are may be 50 years old. You clearly see some aging - mentions of vacuum tubes & slide rules among the interstellar travelers, Martian canals, Mercury tidally locked to the Sun, ...
I didn't find any of these elements coming in the way of the story. Essential element in his stories is weird world & its understanding. That is not likely to get dated anytime soon.