I'd like to hear you speak more on this specific point actually. I'm not sure what you mean, and it's largely confusing to me.
4) It is inappropriate for the scale of the game. If you wanted to have fights resolved on a tactical map with no stacking: great idea! But when the British Isles are 4 hexes, for example, it does violence to the feel of the game. And it scales poorly with size: the feel is best when you have a lot of room to maneuver, but the game design harshly penalizes large empires and maps, favoring smaller ones where the stacking limit performs the worst.
Again, I'm a bit confused by this point. I haven't experienced any of the difficulty you're referring to in regards to scale. Perhaps specific examples would help?
4) It's prone to artificial tactics. Once these are widely known the claim that combat is now more "strategic" will be falsified - because it's false. There is a reason why wargames abandoned the "I move and attack, then you move and attack" mode. It's because it rewarded unrealistic tactics, like soldiers darting from building to building and never getting attacked when they cross the street. Modern wargames have things like opportunity fire (e.g. when you move in range of my city, or artillery, then I attack you *first* as you charge at me.) Civ 5 has taken the worst aspects of the alternating turn approach and amplified them - for example, with cavalry which not only attacks first but which can retreat, or with insta-heal combat promotions. To eliminate the extreme distortion of "all my units attack, then you go" it's important instead to give both sides a chance - in other words, if you can damage someone else then you can be damaged yourself when the other guy gets a move. It's basic wargame design, and it was ignored. Civ 5 fails as a compelling wargame because it didn't pay any attention to decades of lessons from the tabletop world (I'd bet the Civ 5 team is utterly unaware of the principles behind the boardgaming renaissance led by German designers like Reiner Knizia, for example.)
I'm just going to say that your first few sentences are misleading. All wargames are false representations of what actually occurs. You've simply decided that Civ5's representation is inadequate.
Also, I think with this point you misrepresent what the Civilization franchise is. Combat is certainly an important aspect of the series, and it's good to see that they're taking time to change it up and make it more interesting with each installment, however, it's not the point of the game, and never has been.
Civilization is about building up an empire that, to use an overused phrase, is to stand the test of time. There are five ways to win the game, and only one of them requires combat.
5) The new problems created with 1 unit are worse than the big stack problem they solved. No stacking favors big units over little ones, replacing "stack of doom" with "unit of doom". Large armies create gridlock, and the absurd consequence that you can't even use most of your units because you can't even reach the field (in a battle on the size of a continent). This is especially a problem for the AI, which gets clogged and paralyzed with gigantic numbers of units at the highest levels.
This is basically the same point as #2. You're given a system and rather then using it to the best of your abilities, you simply complain about it. What you're saying is essentially true, but rather then just dwell on how stupid you think it is personally, work with what you have to achieve success.
6) The above problems cripple the AI as an opponent. It isn't that they didn't try, it's that the problem they're giving the AI isn't soluble. A bigger army can actually be worse than a smaller one because it can't move: that's very hard to program.
I would say that there are obvious AI problems, and I hope to see quite a few patches come down the pipe in the future. I'm more or less pleased with the foundation they have in place, and I'll probably enjoy playing Civ 5 for years.
Solutions? A modest stacking limit, either with overstacking allowed (but only the "limit" worth of forces permitted to engage in military action) or the AI coded to keep itself 1 or more below the limit at all times (to allow movement.) Unlimited stacking for civilians. Combining units to create armies (and attaching generals) would be a cool idea that would work well. Ranged units can still attack more safely (but don't need to be able to do so from 2-3 hexes), and weak units can be guarded by strong ones in the same hex (a godsend for the AI.) There are plenty of answers to the Civ 4 problem, and unfortunately the Civ 5 model isn't the right one.
I believe, fairly strongly, that this thread is essentially a knee jerk reaction to an unfamiliar system. Give it time, play on a harder difficulty setting, and have hope that the Developers will release more competent AI programming.