There is no single aspect of Summertide that piqued my interest, but a combination. As I mentioned, it falls into the sub-genre of hard science fiction, whose "science" is generally based in some way on modern theories. Given the distance into the future Summertide takes place, this is difficult to apply to much of the technology involved, but there is enough that it has the feeling of possibility.
There is also the milieu of alien species, which is certainly not new to me in science fiction, but the range of shapes was. No species is humanoid save for Humanity itself. For somewhat whose introduction to science fiction came in the form of Star Trek and Star Wars this was something very special indeed.
Finally there is the Builders, the ancient and unbelievably advanced species and the mindbogglingly enormous artifacts that they left behind. Sure, Babylon 5, had such beings as the Vorlons and the Shadows, but while their motives might seem enigmatic, they remained clearly active participants in the interstellar scene. The Builders are never truly revealed and even their sentient creations have little useful to say about their creators' nature. This mystery is the basic premise of the series.
Interestingly, there is no real antagonist in any of the books. The conflict has more to do with the dangerous situations the environments present than anything else. One might argue that the Zardalu (an alien species in the series) were at times the antagonists. In two of the books this is in a way literally true, but I found them presented more as players in a scene written by others (namely the Builders). They acted antagonistic towards the protagonists, but they were never the main issue. The result is that throughout the series the theme is not simply one of overcoming other individuals (though that does happen), but one of exploration and learning.
As for the author himself, Sheffield may not be the greatest author I have read. His narrative lacked the flowing introspective style presented by authors such as Stephen R. Donaldson or the sweeping epic style of those such as Glen Cook, but it was not a bad style, either. His writing was competent and his characterisations were convincing. He understood how to write about people and given them personalities that felt real and possible to empathise with. This is really basic stuff when it comes to writing fiction, but I have read several books whose authors proved unable to do this (or unable to do it more than once, merely repeating the same characterisations under different names in each book). Basically, I never found myself at odds with the author, while at the same time wishing better for the story. Charles Sheffield was simply a good author.
Over a decade after reading the first volume, I have finally read the final one: Resurgence. I purchased the entire series via WebScription.net and read it start to finish. It is just as good a series as I remembered it.
It is unfortunate that Sheffield died the year the final volume was published. It ends with a newly discovered threat and a inference that a new set of protagonists would take over. I have to wonder if this was Sheffield's own indication that the series should continue with new blood directing it, knowing that he might not be able to do so himself. The series never gained the popularity that such iconic works as Isaac Asimov's Robot and Foundation series had, though, so such a future is in doubt. Ultimately, everything is left to the readers' imaginations.