Could someone explain to me why the concept of a one-world government freaks people out?

Now granted, my first exposure to the concept of a one-world government was Star Trek. And yes, if the government was ran by somebody like Hitler or Stalin, it would be a bad thing. But would it really be bad if the entire planet was run as some sort of parliamentary democracy/republic that had, at it's core, a respect for all people, and all faiths? In essence, a code of basic human rights and dignity? If the planet was run by an elected council, with no one person being supreme leader, and had a constitution that expressed the right for all to live and worship as they saw fit, as long as they caused no harm to other persons, this would actually be a good thing, i would think.