The aiming system in Xcom works like if you mix a board game and realistic calculations together to give it a realistic flair but in a way where it doesn’t make much sense. In result it is neither simple nor realistic.
As long as you see the enemy and you do not use a short range weapon like a shotgun, the distance is completely irrelevant for the chance to hit. If the enemy is only 3 meters away or 30 - doesn't matter. Like with a board game mechanic: Is in Line of Sight? Yes/No. As Easy as possible.
And the only cover counting is the one next to your target (or next to you if you are shot at). So as long as you have a car next to you (and somehwat between you and your opponent) you are in cover. The other 5 cars between you and your opponent do not matter at all. (Like in a board game, you want to have as few modificators as possible to keep calculations easy). The whole thing is then expressed in a percentage value, which might be slightly modified by other circumstances but which also somewhat implies to be realistic.
That results in:
Lets say you have two opponents, the one is 30 meters away, and there are 7 cars in between you and him, but he is not directly next to a car, you can basically perfectly hit him. Because the cars in between don't count and distance is irrelevant.
The other opponent is 3 meters away. But he has a car infront of him. You will hardly be able to hit him.
Not only that, because the first opponent has no cover, it is not only very easy to hit him, you will also very likely hit him critically.
And on top of that, due to missing information about how far your guys can see and what is in their Line of Sight, the targets you can hit best, are the hardest you as a player can determine as valid targets, which together with the two step movement system, which does not allow any corrections, leads to wrong assumptions ending in death, especially if you are the potential target.
All game systems in XCom are done in a way to make every little step matter as much as possible. So it is extremely important to move exactly to the right tile. Moving to the wrong one, might either result in not being able to shoot at the target, or resulting in certain death. But the game does not provide much information to help you with that (no line of sight indicator until it is too late).
In the common "best case" you have an 75%-80% chance to hit an enemy. Which sounds pretty good. But there is still a 20%-25% chance to miss. And if you miss, this might result in death of your character. Same goes for the case where you are in perfect cover. A chance of 25% to get hit anyways is still very high. Due to the high damage (especially with crit chances) and low hitpoints these probabilities then might cause a chain reaction. First resulting in panic where your soldiers might shoot each other, leading to death of your squad, failing the mission, upward spiraling into putting the whole campaign into the trash.
And I haven't even mentioned the problem of occupying multiple tiles if hiding behind a wall (enabling you to shoot around corners), lack of info for overwatch, lack of overview due to graphics and especially heights, that aliens always get an extra move once they are discovered countering careful advancement (Solders, which did their move can't react anymore, if you set them on overwatch before, they might not be able to do anything even if aliens are discovered, and if aliens are discovered from a bad angle the double movement can lead to flanking and therefore instant death) and providing a playing field where both players play with different rulesets.
I don't have a problem with hard games. Actually I love nuts which are hard to crack. But if I lose that should be because I made a mistake. In XCom you either lose because of bad luck (you roll a 6, you die), or because the information the game provided was insuficcient. In comparison to that there is hardly anything you learn for the next time.
That said, the staging of the game is great. And the combat was never a real strong point in the previous games. It was more about "Now I got this technology, let's see how it works in combat". So if you are seeing it like that, the situation isn't too bad but of course it also doesn't offer a big sandbox as they previous games did.
Personally I'd have loved to have it work in Ironman mode. But the game (like the previous Games, just in a different way) just isn't made for this.
This was one of the latest "downfalls" in one of my rather short lived campaigns. And it's rather typical for how XCom plays:
http://www.youtube.com/watch?v=C6Q8IUg4TAI
->A shot which looks like a good hit only has a 50% chance and fails. Well it looks like a good shot for XCom mechanics. In reality I am shooting through a wall.
->The Thinman doesn't shoot back as expected but runs directly next to my people just to shoot someone in the back who is very far away and could not have been saved in any case. Of course it is a crit resulting in an instant kill
->Upon mpoving to the thinman I pass a position which is apprently visible by the alien on overwatch (which can't really be determined by the player at all)
->And of course upon attacking the thinman my soldier does the least amount of damage
What you don't see here is that the thinman also dies and the poison goes upwards and kills the guy with 1 remaining hitpoint. I might have guessed that one beforehand though, but doesn't really matter. Mission and campaign was lost on the first death anyways.