Though many might consider this a minor point, I think that this is really one of the "wow" factors in strategy games, both real time and turn based.
In Empire: Total War, when you finally get the rank fire upgrade, the performance of your line infantry on the field takes a drastic turn for the better. You can actually see and feel the difference on the field of battle.
In Civ4, when you built a wonder you could see the wonder in your city. When you researched a new commodity, it would have its own little icon and appear in your trade screen.
The scale of these examples are rather drastically different, but the key point is that the difference is noticable in some way. What are the worse type of upgrades? "Archers deal 10% more damage." I always hated those. Sometimes they were important, maybe even necessary, but I never felt a sense of enjoyment or accomplishment in getting that tech.
I'm not saying don't do those types of upgrades, but I'm saying make it noticable in someway. For example, with the archers, give them a new type of quiver on their backs, or make their arrows glow, or give them a faster animation so an upgraded archer army actually looks like it outlcasses a regular one. When you can create a new type of unit, add a building to the city that represents that fact. Archers get a archery field, foot soldiers get a barracks, etc.
Now, I know that this is a lot of busy work for minor details, but it's the difference between "good game" and "wow, this game is awesome." Displaying this all in the engine might be too much work, especially with all the city stuff, but perhaps you can do a "cloth map" city?
Anyway, just my two cents.
