One specific thing I notice is that many English pop/rock singers, especially in the 60s (don't know about today), tried to imitate the American "R" (a rhotic R, for all you amateur linguists). The problem is that they never quite get it right - and I'm not sure exactly why it sounds off, but it does. I can also hear it when my posh English friend puts on her "American" accent, and occasionally with Hugh Laurie as Dr. House. It's as though it just doesn't sit right in their mouths - they're just not used to making that sound.