Going back to the original question:
I can't answer for others obviously, but the reason my HOF game (Emperor 1st) doesn't have all improvements like factories, manufacturing plants, solar power, etc, is that I usually sell them off after I build my SS. I sell off defensive improvements like city walls, barracks, ports, SDI, etc too (if I even had them to begin with) as soon as I'm the only real power left.
In late game, for a high score, I then go on a buying spree to make sure each city has a courthouse, marketplace, superhighways (good for happiness and better trade/future tech rate). The more happy--ie light blue--citizens you have, the higher your score. Shield production doesn't affect your score.
As for developing the land, I don't bother with mountains, or even deserts unless I have LOTS of spare settlers, which is seldom the case for me. Frankly, I've never had a city over size 35 or 36. Usually 207s to 30's, and quite a few in their teens by game finish. That particular game, I actually finished 20 turns too soon as I got bored. Guess I'm just sloppy that way.
Haven't yet played MP though. I'm very curious to see how that will improve my game overall.