OK. My feeling is that with Writing you can either (a) build a Small Wonder which lets you construct spies or (b) found embassies. Either way, you can immediately use spies/embassies to investigate foreign cities-for a price and with a chance of failure. Also, you have the ability to try and assassinate any great people/Super specialists in the city. Once you get alphabet you can steal techs; when you get map-making you can steal maps; when you get military tradition you can steal their battle plans; when you get construction, you can sabotage their city production; when you get currency you can steal their money and-once you have either nationalism or communism-or some such-you can start to incite revolts and/or bribe units (very, VERY expensive though, and they become barb units, not yours). With biology you can poison a city's water supply (-'ve health). With printing press you can spread propaganda (-'ve happiness, increased chance of a city switching loyalties); with explosives you can start blowing up city and terrain improvements. As you get into the modern age, you should have the chance to 'damage' enemy units, and release chemical, biological or nuclear attacks. Lastly, it should also be possible to use spies-in the capital-to force civics changes (again, VERY difficult and expensive).
The chance of success should depend on several factors, including the mission performed, how many spies you have in a given city, the number of spies they have in the city, the level of diplopmatic relations, the size of the city targetted, any tech-level differences between the civ, and the various civics each civ has (propaganda is much easier in a civ with freedom of speech than in a police state, for instance).
Anyway, just some thoughts.
Yours,
Aussie_Lurker.