I don't have the time to read through the whole thread now, unfortunately, but I have some quick thoughts:
The fallout combat system is far superior to the now prevalent action rpg approach imo, (though I must say that the addition of a ctrl key lock on was very very welcome in DD, as it certainly decreased the twitch factor).
I'm not so sure that a fallout style combat system would suit DD though. In my mind the difference between a turn based system ala fallout and a real time twitch based system ala DD is the focus on roleplaying. No offense to DD, but fallout was a deeper rpg imo. DD is certainly more expansive, but it takes a more traditional dungeon romping style where you go to town to get a quest and trade then leave town to kill stuff. This style would be disrupted by a fallout style combat system because it is so much slower.
The beauty of the turn based system in fallout was that I was very attached to my character and I let my imagination fill in the blanks for me a bit. So, when I was playing fallout 2 and using the unarmed evasion skill, I could imagine my character going jet li, flipping about to evade bullets/melee strikes. This makes it so that the individual battles are more memorable, but this approach demands ALOT less combat. It's a trade off. Real time combat doesn't allow for all of the technique possible in turn based, but turn based is never going to be as exciting. No constant zig-zagging back and forth to avoid projectiles while you rush in to strike the killer blow with an axe.
I think these two different combat engines only fit in with entirely different game styles. The same type of player that loves the slow paced/strategic combat in fallout loves the endless dialogue puzzles of fallout and everything else that makes the pace of fallout snail like compared to a game like DD.
I am unable to play evil characters, I just don't tend to enjoy them. Nevertheless, I would occassionaly dabble in evil while playing FO, and this is another key difference. Imo the fallout combat engine isn't just fun because it's strategic, it's fun because it's evil. The first time I got into a fight in shady sands I ended up crippling Tandi, ("blowing her knee into the next town"), and then I approached her and finished her off with a bullet to the face as she lay helpless on the ground. This is where I think that the FO combat system involves more roleplaying, but I don't see room for this kinda action in DD really. The same is true when my character runs in front of a child during an urban gun fight to absorb the minigun spray that was directed at my companion, and that would've torn the kid to bits.
I guess I'm showing alot of bias here, though unintentionally. I think the DD universe could support a fallout type combat engine, but I think that you'd be missing out if all you went for were the increased strategic options that would be possible. I think a deeper game with more scripted events and quests for more character types, (not class based quests, but "karma based quests", if you will), would help make the turn based combat system much more poignant.
You have to include truly evil, not just "tough guy" dialogue options, and the quest/scripted events to go with, before you can fully enjoy the imaginative role playing options that a turn based combat system allows.
