Greg Mitchell, who frequently writes on Manning, Assange, and WikiLeaks related issues, compiles a list of things whose public revelation he attributes to Manning's action. It's long, but here's a sample:
First, just a very partial list from "Cablegate" (excluding many other bombshells that caused a stir in smaller nations abroad):
....-Yemeni president lied to his own people, claiming his military carried out air strikes on militants actually done by U.S. All part of giving U.S. full rein in country against terrorists.
-U.S. tried to get Spain to curb its probes of Gitmo torture and rendition....
-State Dept memo: U.S.-backed 2009 coup in Honduras was 'illegal and unconstitutional.'”
-Cables on Tunisia appear to help spark revolt in that country. The country's ruling elite described as “The Family,” with Mafia-like skimming throughout the economy. The country's First Lady may have made massive profits off a private school.
-U.S. knew all about massive corruption in Tunisia back in 2006 but went on supporting the government anyway, making it the pillar of its North Africa policy.
-Cables showed the UK promised in 2009 to protect U.S interests in the official Chilcot inquiry on the start of the Iraq war.
-Washington was misled by our own diplomats on Russia-Georgia showdown.
-Extremely important historical document finally released in full: Ambassador April Glaspie's cable from Iraq in 1990 on meeting with Saddam Hussein before Kuwait invasion.
-The UK sidestepped a ban on housing cluster bombs. Officials concealed from Parliament how the U.S. is allowed to bring weapons on to British soil in defiance of treaty.
-New York Times: “From hundreds of diplomatic cables, Afghanistan emerges as a looking-glass land where bribery, extortion and embezzlement are the norm and the honest man is a distinct outlier.”...
-Shocking levels of U.S. spying at the United Nations (beyond what was commonly assumed) and intense use of diplomats abroad in intelligence-gathering roles....
-U.S. used threats, spying, and more to try to get its way at last year's crucial climate conference in Copenhagen....
-Hundreds of cables detail U.S. use of diplomats as “sales” agents, more than previously thought, centering on jet rivalry of Boeing vs. Airbus. Hints of corruption and bribes.
-Millions in U.S. military aid for fighting Pakistani insurgents went to other gov't uses (or stolen) instead...
-As protests spread in Egypt, cables revealed that strong man Suleiman was at center of government's torture programs, causing severe backlash for Mubarak after he named Suleiman vice president during the revolt. Other cables revealed or confirmed widespread Mubarak regime corruption, police abuses and torture, and claims of massive Mubarak famiiy fortune, significantly influencing media coverage and U.S. response.
To be sure, neither most U.S. media nor actual Americans care about any of that stuff. But maybe they should; at the very least, it's good they have the opportunity to know it, if they care to look.