Besides the whole uplift concept, I've also liked the concept that the galactic civilization is really old - like hundreds of millions if not billions of years old. Some of the ships flying around are tens of millions of years old.

I have to say the second and third books, Startide Rising and The Uplift War, are some of my favorite sci-fi novels of all time. The first book is David Brin's first novel and it kind of shows. It's not a bad book, but it's a bit weak. The last three books are a trilogy and they start getting pretty weird and I didn't care for it as much. The second and third books are standalone so you can read them without the first book very easily. I actually had read the second and third books a few times before looking into it a bit more and realizing there was a first book.