Icelandic is an interesting airline: they're not nearly as polished as British Airways, Virgin Atlantic, KLM or Lufhansa, but they do well for budget travel. Their catch is the stop in Reyjkavik (which can involve a no-penalty stay of up to three days, I think). If you're willing to have the connection, they're okay - just don't expect a red carpet treatment (though they are friendly).
As far as Alitalia is concerned, beware: they're notoriously unpredictable and financially unsound. Their ownership has been in chaos for years, and various parts of their workforce are almost always in some sort of slowdown/strike mode. It's not pretty, and I'd avoid using them if at all possible.
I've flown overseas on Continental and United in the past two years, and both have been fine. They're not as refined as their European competition, but they were both very friendly and professionally run.
I've also flown B757 service to Europe (EWR-BRS and LGW-EWR, both on Continental) and it's been fine. The range on these aircraft is similar to that of the B777-200, now that the 757 equipment has been refit with blended winglets for improved fuel performance. Sure, they're not widebody jets - closer in look and feel to the B707 and DC8 long-range flights of the 60s and 70s - but these jets can fly into smaller airports that offer different gateways to Europe. For example, Bristol, England (BRS), can't take full-size widebody jets, yet can handle a long-range B757.