I can't really speak for the older Civs having not played them, but Civ 5 compared to ES is fairly obvious.
Endless space: Cards are mostly cosmetic. The best strategy is to modify ships with heavy defense and a moderate amount of two weapon types (can apply all three in late-game). The choice of customizing your ships is largely an illusion, so from turn 1 until turn 200, your ships are exactly the same. Just with larger numbers and different models. The killer is that "paths" are set, and nearly all fighting takes place at a single system or two. This removes several strategic elements, such as troop positioning. It breaks down into "put my stack of doom on enemy stack of doom. Whoever has higher numbers wins".
And that describes the game as a whole. Nothing changes throughout the game, just an illusion.
So, Civ IV? Isn't that the best game ever around these parts?
Anyway, you're grossly oversimplifying. You might be able to get away with just about any ship on Normal, but you aren't going to get away with that on Impossible or Endless. Not since the Automatons patch, anyway. The AI isn't quite as stupid as it once was, though admittedly, it's still pretty bad. But bad AI seems pretty typical for 4X games since the complexity is so high.
Of course, the end goal is to make a bunch of Dreadnaught-class ships with über armor and weapons and then support them with some buffed up Battleships or Destroyers, but so what? There are constant end-game units in Civ V, too.
Civ 5: A host of different units with different functions. Ranged, cavalry, siege, naval, air, etc.
If your coast is being bombarded by naval units, you cannot just take your stack of "generic" army and right click. If you march regular infantry towards an enemy base without any air support, you will end up taking a lot of cheap hits.
Eh...
Your archers, artillery, etc. can definitely destroy naval units. Those are generally part of the "generic army". Air only matters in the last 1/4th of the game. Don't get me wrong -- I love the tactical, 1UPT aspect of Civ V. I think that it's a vast improvement over the stacks of doom on Civ IV. But I'm not going to hold ES to that standard because it's not the same kind of game. How would something like 1UPT even work in space?
ES could definitely improve upon the system invasion feature, though. Then, we could have ground combat where tactical gameplay makes more sense. That's one of the possible features being discussed for the first expansion. I hope it gets in!
The strategic elements missing from ES are present: If the enemy is bunkered down on hills in front of a fortress, you can move your military around and strike from the back. If the enemy is sending troops longer distances, you can ambush reinforcements from the middle, or cut off retreats. You can actually manipulate the battlefield and map.
There's some of this in Endless Space, though. If you see a ship heading for a specific system, you can meet him there because you know that he can't turn around without first reaching that system. If you see a wormhole, you can position a fleet there to ensure that nobody gets through. You can strategically position fleets on enemy systems to blockade his access to key resources.
I guess my overall point is that the combat in ES isn't amazing, but then neither is the combat in Civ V. In fact, combat is probably the worst aspect of every 4X game that I've played. That's probably why I prefer non-domination victories for the most part. But if you're into the other three Xs (exploration, expansion, exploitation), then ES is a pretty decent game at a pretty decent price. Or, it was. $15 is a good price for ES. $30 is probably asking too much.
