For example, in order to make the right decisions when dealing with foreign affairs, a government needs accurate information from its diplomats and other agents. Already one diplomat (the US ambassador to Mexico) lost his job because he provided an honest assessment of Mexico's organized crime problem and the ineffectiveness of Mexico's attempts to deal with it in his reports back to Washington. Wikileaks leaked those reports, and that embarrassed Mexican officials. Ambassadors can't function if the host country is pissed off at them, so the Ambassador was withdrawn.
The major effect of these leaks is going to be for the government to become less open. Assuming Manning is indeed the source of most of these leaks, the reason he had access to all that information was because the government was trying to be more open internally. One of the findings of the 9/11 investigations was that collectively we had enough information to anticipate an stop the attack, but because the information was spread among several agencies, no one was able to put it all together and see the connections.
In response to that finding, the government purposefully started making information more widely available across agencies, and to more people in those agencies.
Surely that will be cut way back now, with much less sharing between agencies, and information only going to those who definitely need it.