This is what I've observed out of how Americans act, and how people react to Americans. Not really my opinion, but mostly common complaints. Keep in mind this is a massive generalization.
People from other countries don't like Americans because the ->average<- American finds it really difficult to look outside of his/her country's borders and never stops being shocked by the fact that other countries and cultures actually exist, and celebrates its sub-300 year history as if it's the most significant thing on the planet. The average American has a lot of trouble thinking of any cultural/social issue in a context that isn't in the USA, and despite having the world at their fingertips in 2015 this isn't showing any signs of improving. Americans find themselves in awe every time they find out that different places do things differently, yet their superiority complex stands firm.
In their confidence and arrogance, the average, patriotic, g*d-fearin' American has no problem taking a steaming **** on every other nation, because they view them as a competitor. Then there's the opposite, where their strong enthusiasm for another country is very embarrassingly misinformed, alike to how Japanese people don't like weeaboos. Being English, I cannot stand Anglophiles from places like Tumblr.
To other countries, like all those within the European union that know lots about each other, this makes America appear very, uh, undeveloped. So you can see why people don't like it. And remember we're talking about several very different countries in terms of language, culture, politics, and documented history spanning thousands of years. Europe is not one little country.
Where the USA has spent so much time in isolation, tons upon tons of **** was happening in the world, and where hundreds of different nations and societies were constantly interacting with each other, the American populace has kept relatively very little perception of the rest of the world outside of "those guys". America's rampant xenophobia and propaganda only started slowing down after the fall of the Soviet Union. The only other place like this that I can think of that isn't an underdeveloped mudhole is North Korea. And maybe possibly Switzerland, they like their isolation.
America is a major superpower, it's powerful, wealthy, influential, and in spite of this, it hasn't helped its people become more informed, it doesn't actively encourage an interest in other countries or cultures outside of its own.
In all fairness, Americans I have met in person have always been very keen to hear about where I'm from and would ask all sorts of questions with eagerness. Most Americans don't actively seek any knowledge because they've never had the pressing need to do so, and when they do, it's usually some kind of hobby for them, or it has something to do with their job or their family.
But America, as a generalized mass/nation, should consider doing more for its citizens' awareness of the world. Even their politicians have a skewed perception of the outside world. Despite thousands of years of history outside of America that Americans can learn from, their preferred point of reference is a sub-300-year history of the USA.
TL;DR When I hear people complain about Americans, it always comes down to more or less this: People don't like America because it's like being in a room with lots of people, but the one wielding the big stick is also the only one with down's syndrome and everyone else has to put up with him. The kid with down's is too busy getting excited about the big stick to acknowledge who he shares his space with.