I have just read this entire thread in one sitting, and in addition to some interesting reading my eyes really hurt!
I am another of those that has played every computer Civ since number 1. I do not like the RTS genre at all, rather despise it in fact. I do like the Total War series. So, disclosure aside, I do not, at all, like the 1 unit idea. I have several reasons why, and although I am sure they will not make one bit of difference to anyone there would be no need for a forum if I wasnt supposed to spout it out anyway!
1) Unless the resource/unit restrictions are ultra severe I foresee that this will result in
more units on the map, certainly not less. Why? Because you will now be rewarded for having the conventional double line of units along your boarder. How else will you stop an invasion and protect your country? Think of Avalon Hills Third Reich. What do you end up with in France? You end up with a map covered with counters. Yep, they are out of the city and covering the map so much you cant see anything else!
2) Supporters of 1 unit seem pretty universal in wanting more tactical combat. Personally I want
less tactical combat. I know that is just a what I want vs. what you want statement, but more than that Civ has never been a tactical combat game. Why start now? What is the advantage? I would even prefer if they had found a way (perhaps like culture) to simulate combat entirely. A system where you built military units and they went into a pool, off map, and when at war would exert pressure against the military of the other Civ. Why would I like that? Because Civ is (to me) at its core a game pitting Civilizations against each other, not a military game or simulation.
3) Given #2 you may be surprised that I like the Stack of Doom. Like many other posters I feel it represents better the hands off of a President than the almost miniatures based combat that is coming. Is it not very tactical, you bet! Again you want more tactical, I want less.
4) In addition to Civ I play and have played a lot of wargames. I have most Avalon Hill and Victory Games titles in my library. From that perspective I foresee that it will be very hard to defend anything. How will you be able to defend your cities when you can only defend with 1 unit and can be attacked by multiple? Will cities simply be changing hands each turn? I know we have not seen the real thing yet, but
(and see 5 and 5a)
5) Among the supporters of 1 unit there seems to be a dislike of the fact (and yes it is true) that in all other Civs a country that can out produce another has a great advantage. Im not an expert in the history of warfare, but isnt that generally the case? Unless there is a real disparity in technology wont the side that can out produce win in the end? Certainly there is a will to fight or superior tactics argument to be made, but assuming these factors are equal who can provide a number (not just 1 or 2) of examples of where the similar lower producing country won? I am pretty sure that in most cases the country that can deliver the most resources to the field of battle wins in the end.
5a) Assuming you dont like that, how is the new system going to change that? If there is this great limit of units that some speak of, and the larger Civ can field 20 units and the smaller can only field 10, how is the smaller supposed to have any better chance with 1 unit than with the old system? At least with the old system the smaller Civ could stack up a chokepoint, or stack up in the city the bigger Civ was trying to take. If you are looking for more parity amongst Civs of different size I dont see how 1 unit is going to help with that, to the contrary I would think.
6) While many supporters of 1 unit dont mind the seeming inconsistency of the hex scale, or say that you cant complain about the hex scale without complaining about the movement scale, I respectfully disagree. The hex scale should be the hex scale. If it is 100 miles or 10 miles, or 1 mile, Archers should not be able to shoot 2 hexes! Oh you say, its just a game! Does that mean that we have to have unlimited Liberty Hall? If so why stop at 2 hexes? If Archers can shoot 2, why shouldnt Cannon shoot 5, and Artillery 20? If you want a tactical game, have a tactical game, not a mish-mash.
6a) I also disagree with the movement rates. Even given the large scale they should be higher. I can accept that in the earlier times movement is not just limited by how far a unit could literally walk or ride or sail but the need to be able to support them and keep Command, Control and Communications with it and that those considerations are abstracted in slow movement. By the modern age though those considerations no longer keep pace with the slow movement of the units. IMHO Also to some posters points, when the game includes multiple scenarios where the map is the Earth I think it is fair to say that the scale of the map is intended to represent the whole planet, and one the size of Earth. I really dont see what argument can be presented to refute that (but I am sure someone will!

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7) So, in conclusion, my personal opinion is that 1 unit is bad. I dont like it specifically because it looks to add more tactical combat to a game that I am not looking for tactical combat in. I believe that in many cases this system will make the situations some complain about with SoD worse. I think it will add, especially to the end game, a vast amount of tedium and turn-by-turn micromanagement (while we dont have the system yet that is what I believe will happen). In fact other than nicer looking graphics I am sorry to say that I have not read or heard of even one new feature (at least that comes to me at the moment) that I like about Civ5. As I have always been such a fan of the game that disappoints and saddens me. Unless it turns out that the implementation is so much different than what seems to be coming I doubt I will purchase Civ5, a statement I can hardly believe I would ever be saying.
8) Not intending to offend it almost seems to me that many of the supporters of 1 unit would really prefer Civ as a RTS. Or am I missing something? What kind of game are you really looking for?
9) I know I am not going to change minds, but if I have seriously misrepresented someone's point I would like to hear what was intended.