Don't say "I don't have any weaknesses." That sounds ridiculous. Take something that was bad and say you're improving. Like... "I used to wait until the last minute to do homework, but I realized that working in advance makes much more sense and reduce my stress, so I've been improving in that area. "
But it comes off as hating all Americans, because you usually imply we all fit under your stereotypes.
I just don't go posting blogs like, "Man, those Brits sure do have bad teeth! I'd hate to be a dentist over there - they clearly never go to one!" It's offensive, and even if it were true of certain individuals, it's still rude to go around waving my flag of "Everyone sucks but me!"