In any strategy game, the developer must always strive to increase the importance of strategic choice. For example, because distances are large in ETW, it is an important decision whether or not you want to station your navy in the New World, India, or Europe. Fleets can not be everywhere once.
Similar logic should have been applied to upkeep costs. The ratio of land unit cost to upkeep cost is far to low, typically ~2:1. This means, that when a unit is lost in battle, it takes only two turns before you can afford to recruit another of the same unit. This is disastrous for strategic decision making as it dramaticaly lowers the stakes of each battle. When I was prussia I captured Saxony in a major battle, losing 90% of my army. However, within 2 turns my force was back to full strength. If the ratio of unit cost to land unit cost to upkeep cost dramatically, say to 8:1, then my decision to risk major loses in battle would have a much longer impact on my countries military viability -- its ability off attacks from other countries.
Moreover, increasing the ratio of unit cost/upkeep cost would increase the long-term decision making regarding raising an army, and it would allow small countries (like Prussia!) to have larger standing armies then major nations (like France) if they spent many years building an army (of course France would be capable of raising a much larger army, but it wouldn't be able to do it in a single turn). As is, countries reach there 'carrying rate', or the maximum amount of units there country can support, within only 2 turns.
Perhaps this is why navy combat is more (strategically) interesting. Building a fleet takes a long time, and is relatively expensive (ratio ~6:1) relative to its upkeep costs. This means every ship is an investment and is a major choice, and you cannot become a sea power in two turns.
It would be very simple to fix this issue (not sure how much AI coding would be required to make it understand the change, probably not much), and it would be far more interesting. I would also be far more interested in how my army faired in battle.