Contrary to most review, i really love Civilization 5, it is pretty neat (at least for single player) however it is not perfect so i tried to play another games so i get Endless Space.
Endless Space is one of those games that is easy to learn but hard to master.
First, is the races, races are pretty unbalanced. In Civ5, some races are more easy or hard to win, in ES, it is more radical, pick a wrong race and you are pretty screwed.
Second is the research tree it is too uninspired, fast and some techs are a must, unbalancing the game in a couple of turns. Also, most techs are not self explanatory. Also, the fact that the research tree is big does not help at all. In CIV5, the research tree, while important, it does not unbalance the game unless the player is too advanced.
Third, is the population, in CIV5, cities can growth almost unlimitedly. In ES, is pretty easy and fast to max the population of a planet, and it is not possible to create a new planet. So, to explore new systems is a must and achieve a technology to populate harsh planet. So again, tech is everything.
Fourth, battles are pretty unbalanced, an army can be destroyed without even generating any damage. In Civ5 happens the same, however, it is possible to destroy a strong enemy sending consecutive waves of outdated forces. Again,in ES tech is everything.
IMHO, this game is fine and nothing more.
I would agree with many of your points except the idea that he game is hard to master. Since there's really no strategy to combat and the AI is completely incompetent and will never actively adapt to counter you, simply loading your ships primarily with missiles (early game) or beam weapons (mid-late game) and pumping them out like crazy will easily win the day.
There's really no strategy to it at all and one decent fleet with with a good officer and something like 15 command points worth of ships can easily bring down an entire empire with 250+ command points worth of ships without taking a single loss. It's one of those games where if you're familiar with other titles in the genre you'll feel like a master by the end of your second game (if not earlier) because most of the mechanics are so shallow. Early exploration and colonization is fun, but beyond that, the shallow mechanics, broken diplomacy system, and brain dead AI really kill the experience.