There are lots of important things that go into a great cover version - the first is a good song to cover, there have been far too many versions of "Boys of Summer", for example. The song being covered has to have a meaning for the band or artist covering it, otherwise it will just sound like someone trying to steal someone else's success. The really good covers are the ones that people didn't know were covers, such as Annie Lennox's "No More I Love Yous" or Sinead O'Connor's "Nothing Compares 2 U" (originally a Prince song), because the artist singing in it can show a personal connection to the music, and that will always be easy to hear. An example of a really, really bad cover might be Madonna's "American Pie". She's not Don McLean, and the America Don McLean was singing about has nothing to do with anything Madonna is, was, or could ever stand for. That's what made it awful (oh, and the electronic organ thing in the background). The other really important element in a good cover is managing to combine a healthy respect for the original while including the new artist's influences. Oasis have a B-Side version of "My Generation" by The Who which is played with almost too much respect for the original - it sounds the same and we've already had that - Nickelback's version of Elton John's "Saturday Night's Alright for Fighting" however, is excellent, because there is a good mix of both acts. The other extreme might be The Pet Shop Boys version of "Can't Help Falling In Love With You", which jarred badly because there wasn't any respect for the original and the styles just didn't work. It felt like profiteering.