... The main problem for me is its scoring system. It penalizes for playing at higher difficulty. As a result I got my best score (9,500) at about 54%, and at 75% I always do much worse. ... So, scoring system is greatly unbalanced. ...
The others have given you some excellent suggestions.
I have to suggest that the scoring system is not "unbalanced"
; it is intended to force you to learn the fine points of the game in order to get as high or higher a score at the more difficult levels. Why should one be able to score the same with the same effort no matter the level of difficulty?
There are several variations in addition to the difficulty levels that are intended to add interest.
The Victory Condition
"A Place In History" uses the most complete scoring system.
You may wish to check this thread to read about some of the "fine points" of the game:http://tropico.strategyplanet.gamespy.com/cafe/index.php?topic=9134.0