I'm still a bit undecided on the "no stacking" dynamic. It is true that the "stack of doom" effectively unlimited stacks in Civ3, and 4 were "unbalancing" in the sense that they produced moments of sheer terror and moments of unadulterated gloating. When an AI showed up on your border with a stack of 30 or so units and your entire army comprised about 5, it had a unique kind of "pucker of the nether regions" effect that made Civ4 and Civ 3 special. On the other hand, when you assembled your doom stack of 60 or 80 on your hapless AI neighbors border, smiling at him in the diplomacy window right up until that last perfect moment, and than BAM! war decced, and vaporized mere turns later . . . Those were awesome dynamics that gave an otherwise somewhat slow and sometimes tedious game some serious drama and change of pace.
The thing that should be noted: while these "doomstack" events were intense, the AI was fairly proficient with it himself so it wasn't like it was _completely_ all about human exploit (though yeah, that was an issue too).
The overall effect of "no stacking" seems to be . . . well, most notably, that the human is forced to fight "through" the same limiting bottleneck that AI "intelligence" must force it to fight through. However, I'm not even sure if the AI is that good with the no-stack combat dynamics. Mostly he seems to send in forces one at a time, poorly sequenced and not properly arranged to encircle and create force multiplier effects. However, given that the very act of doing this is in itself slow, and not always tenable, this effectively puts the human and AI on something more of a level ground. Its like making Mike Tyson and a 10 year old fight to the death with nerf weapons. Yes, Mike has a serious advantage, but given he can only do the same amount of damage as the kid, his advantages are a bit moot.
Would love to see a mod that struck a "middle ground" on this whole stacking thing, meaning: not "no-stacking" and not "stacking unlimited" but something more like: one unit, plus up to two additional units of alternate combat class (melee, ranged, mounted-melee, mounted-ranged, scout, etc.). Another possibility that might fit with the game's existing dynamics quite well: if you are a militaristic Civ (honor or whatever it is) and you have amassed a pretty high degree of experience and had a couple Great Generals, you can somehow start to generate "Elite Operations" points (or something along those lines). When you generate enough, you can create one from a set of choices. These units would have a number of perks including stacking with any other unit, certain promotions at spawn such as "heal 5 to all adjacent," withdraw from combat, etc., etc.