I'll agree that much of the antiamericanism is unreasonable and uncalled for. But not all of it. After all, there are almost no exemples of powerful groups in History who haven't abused their power; America is right now more powerful than any group ever was.
About warmongering: It's true that America isn't any more trigger-happy than the average Western country. But there were situations where it wasn't on the side of angels (if you'll excuse the expression!). I'm thinking of Vietnam, of Chile - where the CIA helped Pinochet overthrow a democratic regime and establish one of the bloodiest regimes of South American history. You see, the democratically-elect president, Allende, was a socialist (not a communist - a socialist). And I find it interesting that just a year and a half ago, nobody in America seriously considered invading Irak; but now, whithout any new developement on the Iraki side (at least nothing that didn't happen regularly before the WTC attacks), it sounds like America doesn't have any other choice but to attack.
On neo-colonialism: The situation is that in the Third World, the West still has immense influence. Very often, the third-worlder elites are given corruption money in order to buy from the West (America and Europe alike) all kinds of things that they don't really need (espescially weapons). Plus, the poor countries always sell the West raw materials at low price, and then but manufactured goods at high price. On top of it, both America and Europe give their industries and agriculture immense subventions which prevent the Third World from competing.
Then there's the refusal of the Bush administration to comply with the Kyoto agreements (which the previous government signed!) to regulate pollution, the near-inconditional support to Israel (and, more recently, to the war criminal Ariel Sharon), the embargos on Cuba and Irak, and many more. It may not be worse than what other countries do - or would do if they had the power - but it does give people some good reason to dislike America.
That being said, I'd like to say that I do think America is a better place than most of the world, and I do consider moving there one day (I'm an Israeli living in France). But I'm not a big fan of its current policies.