You got my 'mental hold' concept in one, PCarroll, but to be explicit, I would define it as:
the ability to contain and comprehend the current game state.
This could be stated as a percentage, or it could be simply expressed as the required amount of data required to maintain proper gameplay. My primary point, I suppose, is that Dominions 2, on larger maps, inevitably runs into the problem of being out of scope for almost any player. It certainly, in my opinion, gets to the point that missing command opportunities will be more common, and missing optimal plays will be expected. I used Chess as an example because the 'mental hold' required, while not constant, is bounded to a fairly manageable level. Understanding the game state is not a particularly difficult task, particularly for a basic understanding. Extrapolating from that position is the mark of more advanced studies, certainly, and that's where a large part of the skill comes in, but the ability to retain the game state is quite important.
Wyatt