Let me declare this upfront: too many people have a visceral and unfair dislike of all things American. However, the most vicious dislike is to be found among Continental Europeans, particularly from German-Speaking countries. I find their self-righteousness quite unbearable. UK people generally like Americans, altho' the Guardian crowd is a bit different.
Now: having said that, let's keep in mind that there's a bit of a difference between slagging off Americans and Indians and Pakistanis. This is a difference in practice, and not a difference in theory. Theoretically the point that this thread makes is valid. But in practice, there's a chalk and cheese difference because of the vast status difference between Americans who live here and Indians and Pakistanis who do.
Americans run the show in the UK. Americans who did not complete their PhD at a US university get hired as associates by the London offices of I-banks for technical analysis jobs, whereas Brits who did complete a PhD get hired as senior analysts. I have seen this with my own eyes. Americans get paid more than their European counterparts. Americans run the tube, many financial and academic institutions etc. Undoubtedly many of these appointments are on merit. But Europe is a well-educated place with some significant advantages of its own- and in some areas (public transport, education up to the undergraduate level) does better than the US. So there is a sort of worship of all things American by the New Labour crowd that extends to areas where America itself does not have an enviable record.
Hence the resentment of UK people against Americans is the kind of resentment that I feel toward my boss. For me to make jokes about him would, I suspect, be a form of release, but for him to make fun of me would be merely cruel.
By contrast, Indians and Pakistanis (esp. the latter) have a much lower social status in the UK. The same goes for Eastern Europeans. All these countries have a huge amount of human talent- more than the US- but British people generally see them as spongers, asylum seekers and the like.
The same thing applies to Jews. When people have faced the possibility of actual physical extinction (the Jews) or daily threats of racial violence and abuse (ethnic people and immigrants in many areas today) that does take things to an entirely different level. Even if many ethnic people do fine thank you, I think you'll find it hard to come up with too many people who say that they haven't at one time or other been abused or assaulted because of their skin colour. Just the other weekend, the Observer ran an article on the (White) widow of a mixed-race man beaten to death outside a Brighton nightclub and how the assailants walked the streets freely and laughed at her and mocked her when she encountered them. Doesn't happen to the rich Americans living in Earls Court, Knightsbridge, Chelsea etc....or even American students. If an American were beaten to death in the UK or Europe simply for being American, it would be headline news.
The status of Americans in the world today, and the UK, especially is actually extremely privileged. This, I suspect, is the reason why many people don't get outraged by "do you hate Americans" threads, but might by "do you hate Indians" or "do you hate Blacks " or "do you hate Jews"- you simply cannot ignore the reality to which people are responding.