I believe that everything should be on the manual. EVERYTHING.
Including for example the precise function of each wonder, the description of each unit, each city improvement etc etc.
I realize that may be a somewhat... thick manual but I think that when the player is in front of the game she/he should devote the whole of his/hers attention playing not trying to find out what the manual didn't include. (for example what does the republic do in terms of economy and so on).
And having a complete manual of CIV to read at your leasure is a really a welcomed luxury
Also there has to be clarity and easy of use in the manual. Sections and subsections for everything much be clearly categorized. CIV 2 manual is really mediocre. A nice example would be the Alpha Centauri manual.
That is my humble opinion
Christos