The Lensmen series is arguably the first real space opera series. If you are a science fiction fan you owe it to yourself to read it, for historical reasons if nothing else. Not that you should only read it for that, the series is a classic for a reason. While I could not fully describe it here I will list out what is more unique about it, and what sticks in my mind.
One thing that was a driving force of the series was the development of technology, specifically weapons and other systems on ships. One thing Smith understood very well is that the biggest secret of any new technology is that it is possible. Once your enemy knows that something is possible, they WILL find out how to duplicate it, it’s just a matter of time and effort.
The technology and tactics also evolved, new weapons forced new tactics, and new tactics forced new weapons. Very few books show this, even though it would be pivotal in any large scale conflict.
Second the scale of the series was very well done. By scale I mean that civilization (good guys) covered most of the galaxy, but it was not as simple as us having 2/3 of the galaxy settled and that part being safe and known. It was more like we have small number of stars settled and most of the rest mapped. But this is spread through the whole galaxy, so there are planets in the middle of civilization that are unknown, maybe even whole empires that have yet to run into us.