In the Plan of Action issued at the last meeting of the WSIS, they listed 10 goals, which included a single international database of user information and detailed monitoring of internet usage in libraries, schools, univerrities, museums, hospitals and just about everywhere else. They would know what you are doing, where you are doing it, and the security of that information would only be as viable as the good will and competence of the UN, qualities which they are not generally known for. The plan also proposes adapting "all primary and secondary school curricula to meet the challenges of the Information Society," essentially putting the UN and its agenda and curriculum in every classroom in the world. The Plan of Action is a frighteningly intrusive and comprehensive document, unrealistically ambitious, and promising a huge bureaucracy interfering in every aspect of the internet.
In the past UN groups have expressed a great deal of interest in controlling the internet, especially in the are of limiting free speech. The UN High Commissionar for Human Rights has proposed regulating hate speech and "screening racist propaganda." The International Telecommunications Union has proposed sweeping regulation of internet practices and conduct. A number of influential UN member nations have draconian internet regulations, ranging from France's hypersensitivity to certain types of internet behavior and commerce to Iran's practice of throwing bloggers in jail to China's total isolation of its own network and extreme limitations on free speech.
In a sensible world the Internet ought to be a way for the freedom and opportunity of the United States to penetrate into the restricted societies of the world, but under the control of the UN it's far more likely that the extremism and intolerance would become the operating standards of the internet and there would be a serious effort to impose them on American users. With almost 200 member nations, all with different rules on internet usage, the bureaucratic tendancy would probably be towards greater restriction to placate extremists, rather than the freedoms we currently enjoy.
Then there's the issue of UN inefficiency. The technology of the internet changes rapidly and many experts have serious doubts of the ability of a UN bureaucracy to keep abrest of developments and respond effectively to changing needs and priorities. Under the UN the internet might well fragment and function far less effectively than it currently does.