Endless Space is a lovely looking 4X game, but has a number of strategic flaws which, while adding elements to gameplay, don't make sense (and hence, break suspension of disbelief), and don't work in an entertaining manner.
Economy is well abstracted, and there are lots of options for expanding your economic foundations, but there are some factors which are largely out of direct player control -- population happiness is one of them. Happiness affects everythning, as in most 4X games, but unlike other 4X games, you don't seem to be able to invest much into helping your populace be happy. Wars in particular make you people unhappy -- to the extent where an aggressive military playing style is basically out of the question. The game's design limits the options for the "kind" of player you want to be -- you basically must play by the rigid set of rules the developer has laid down, or you are penalized to the point where you simply cannot beat even the normal AI.
Wars must be routinely tempered with diplomacy, and expansion must be limited during growth periods... or your expansion will fail, no matter how large your military, economy, etc. Conquering opposing races is almost out of the question, unless it is done over a protracted period. DIfferent races have different features, in this regard, but the trouble with such a setup is that your playstyle is largely known from the outset, by every other player in the game.
Space combat is visually spectacular, and has some interesting tactical twists, in the form of a rock-paper-scissors tactic system. Ship design, is fairly involved, even though weapons basically boil down to 3 types, with levels 1 throgh 5. Ship designs can vary the amount of weaponry and defenses they have, but the game often boils down to something of a "who researched the right techs vs the other guy" fight, yielding a boring, balanced research mechanism as the most effective in basically any game.
The most severe issue with space combat, however, is that fleet size is limited by elements of the tech tree. You can build a fleet of 50 cruisers, but can't put more than 1-2 in a fleet, plus a couple destroyers/corvettes, without researching the right aspects of the tech tree, and even then, the numbers are quite limtied. Thus, an economically superior empire cannot necessarily win with sheer numbers, as each fleet engagement is limited by the number of possible ships in the fleet -- a small empire, with a fleet of 9-13 ships, with a similarly hard-to-come-by "hero" admiral (also limited to a very small number, unless you increase the number with tech), could conceivably defeat an armada of 50+ ships of the same technology level, based upon these nonsensical fleet-size limitations. As such, increasing your effective fleet size is nearly the most important aspect of space combat, all by itself. While this promotes balance, I would argue that the limitation is so restrictive, as to diminish strategic & economic options to the point of boredom.
The inclusion of "heroes" in interesting, except that you are limited to a very small number of them. It is nowhere near as easy to acquire, or utilize a hero in ES, as it is in a game like, for example, Heroes of Might and Magic. You may have 1-2 heroes that are "Admirals" typically (the rest are typically governors of single star systems), but those heroes may not command any more vessels than are allowed by the fleet size limitation. Thus, the benefit to a hero in a small empire is much larger than that of a hero in a large one -- and large empires can be easily overwhelmed by a sum of smaller nations, simply due to the number of heroes involved, and the fleet size restrictions. Even 2 vs 1 can be almost impossible odds under these conditions, and the happiness issues with warfare.
Diplomacy is limited in much the same fashion as combat is -- by options in the tech tree. A player cannot even offer a peace treaty without researching the idea, first. Thus, much of the diplomacy of a game is hidden until the mid-way point of a game, and even then its fairly limited.
Endless Space boils down to an exercise in walking down the middle of the road. Straying too far in any direction is just an expensive diversion, and won't work against the AI, or in any interesting MP game. No blitzkriegs, surprise attacks, radical technology developments, or diverse battle tactics allowed.